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Document and Entity Information - shares	3 Months Ended
Mar. 31, 2019	May 09, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	GRTS
Entity Registrant Name	Gritstone Oncology, Inc.
Entity Central Index Key	0001656634
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		35,718,873
Entity Emerging Growth Company	true
Entity Small Business	false
Entity Ex Transition Period	true

Condensed Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 34,572	$ 52,183
Marketable securities	97,590	100,927
Prepaid expenses and other current assets	2,838	4,526
Total current assets	135,000	157,636
Property and equipment, net	15,652	29,494
Operating lease right-of-use assets	22,018
Deposits and other long-term assets	2,817	2,428
Total assets	175,487	189,558
Current liabilities:
Accounts payable	3,622	4,825
Accrued compensation	1,694	3,951
Accrued liabilities	803	992
Lease liabilities, current portion	2,574
Deferred revenue, current portion	5,419	5,340
Total current liabilities	14,112	15,108
Deferred rent, net of current portion		1,353
Other non-current liabilities	2	12
Lease financing obligation, net of current portion		10,490
Lease liabilities, net of current portion	17,104
Deferred revenue, net of current portion	12,047	13,473
Total liabilities	43,265	40,436
Commitments and contingencies (Note 6)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; 10,000,000 shares authorized at March 31, 2019 and December 31, 2018; no shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively
Common stock, $0.0001 par value; 300,000,000 shares authorized at March 31, 2019 and December 31, 2018; 29,024,382 and 28,823,130 shares issued and outstanding at March 31, 2019 and December 31, 2018, respectively	16	16
Additional paid-in capital	276,703	275,593
Accumulated other comprehensive income (loss)	67	(85)
Accumulated deficit	(144,564)	(126,402)
Total stockholders’ equity	132,222	149,122
Total liabilities and stockholders’ equity	$ 175,487	$ 189,558

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,347
Operating expenses:
Research and development	15,899	$ 11,401
General and administrative	4,377	2,038
Total operating expenses	20,276	13,439
Loss from operations	(18,929)	(13,439)
Interest income, net	920	63
Net loss	(18,009)	(13,376)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities, net of tax	152	(15)
Net and comprehensive loss	$ (17,857)	$ (13,391)
Net loss per share, basic and diluted	$ (0.62)	$ (6.03)
Weighted-average number of shares used in computing net loss per share, basic and diluted	28,938,891	2,217,726

Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2017	$ 97,282	$ 156,937	$ 1	$ 2,045	$ (74)	$ (61,627)
Balance, shares at Dec. 31, 2017		17,797,529	2,152,525
Unrealized gain (loss) on marketable securities, net of tax	(15)				(15)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	22			22
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares			63,048
Issuance of common stock upon exercise of stock options	6			6
Issuance of common stock upon exercise of stock options, shares			8,194
Exercise of common stock warrants	13			13
Exercise of common stock warrants, shares			40,257
Stock-based compensation	354			354
Net loss	(13,376)					(13,376)
Balance at Mar. 31, 2018	84,286	$ 156,937	$ 1	2,440	(89)	(75,003)
Balance, shares at Mar. 31, 2018		17,797,529	2,264,024
Balance at Dec. 31, 2018	149,122		$ 16	275,593	(85)	(126,402)
Balance, shares at Dec. 31, 2018			28,823,130
Cumulative effect of adopting new accounting standard (Note 2) \| Accounting Standards Update 2016-02 [Member]	(153)					(153)
Unrealized gain (loss) on marketable securities, net of tax	152				152
Lapse of repurchase rights related to common stock issued pursuant to early exercises	17			17
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares			51,314
Issuance of common stock upon exercise of stock options	$ 86			86
Issuance of common stock upon exercise of stock options, shares	149,938		149,938
Stock-based compensation	$ 1,007			1,007
Net loss	(18,009)					(18,009)
Balance at Mar. 31, 2019	$ 132,222		$ 16	$ 276,703	$ 67	$ (144,564)
Balance, shares at Mar. 31, 2019			29,024,382

Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Mar. 31, 2019	Mar. 31, 2018
Operating activities
Net loss	$ (18,009)	$ (13,376)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,036	909
Net amortization of premiums and discounts on marketable securities	(519)	(103)
Stock-based compensation	1,007	354
Non-cash operating lease expense	1,344
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	778	(41)
Deposits and other long-term assets	(389)
Accounts payable	(235)	(642)
Accrued compensation	(2,257)	(1,186)
Accrued and other non-current liabilities	104	(872)
Deferred rent		(103)
Lease liability	(640)
Deferred revenue	(1,347)
Net cash used in operating activities	(19,127)	(15,060)
Investing activities
Purchase of marketable securities	(9,592)
Maturities of marketable securities	13,600	7,470
Purchase of property and equipment	(2,578)	(1,692)
Net cash provided by investing activities	1,430	5,778
Financing activities
Proceeds from issuance of common stock, net of issuance costs	86	9
Net cash provided by financing activities	86	9
Net decrease in cash, cash equivalents and restricted cash	(17,611)	(9,273)
Cash, cash equivalents and restricted cash at beginning of period	53,175	39,999
Cash, cash equivalents and restricted cash at end of period	35,564	30,726
Supplemental disclosures of non-cash investing and financing information
Property and equipment purchases accrued but not yet paid	513	$ 328
Remeasurement of operating lease right-of-use asset for lease modification	$ 1,779

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2.	Summary of Significant Accounting Policies

Basis of Presentation

The accompanying condensed financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”) and the rules and regulations of Securities and Exchange Commission (“SEC”) for interim reporting.

The condensed financial statements are unaudited and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary for the fair presentation for interim reporting. The results of operations for any interim period are not necessarily indicative of results of operations for any future period.

Certain information and footnote disclosures typically included in annual financial statements prepared in accordance with U.S. GAAP have been condensed or omitted. Accordingly, these unaudited interim condensed financial statements should be read in conjunction with the Company’s financial statements as of and for the year ended December 31, 2018, which are included in the Company’s Annual Report on Form 10-K, as filed with the SEC on March 28, 2019.

Reverse Stock Split

On September 20, 2018, the Company amended and restated its amended and restated certificate of incorporation to effect a 1-for-6.9 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock. The par value and the authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements has been adjusted to reflect the Reverse Split.

Need for Additional Capital

The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had cash, cash equivalents, and marketable securities of $132.2 million and $153.1 million as of March 31, 2019 and December 31, 2018, respectively. The Company had an accumulated deficit of $144.6 million and $126.4 million as of March 31, 2019 and December 31, 2018, respectively. The Company had net losses of $18.0 million and $13.4 million for the three months ended March 31, 2019 and 2018, respectively, and net cash used in operating activities of $19.1 million and $15.1 million for the three months ended March 31, 2019 and 2018, respectively. To date, none of the Company’s drug candidates have been approved for sale and therefore the Company has not generated any revenue from contracts with customers. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these condensed financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the condensed financial statements and the reported amounts of revenue and expenses in the condensed financial statements and accompanying notes during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to revenue recognition, clinical and preclinical study trial accruals, fair value of assets and liabilities the present value of lease liabilities and the corresponding right-of-use assets, and the fair value of common stock and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.

Other Risks and Uncertainties

The Company is subject to a number of risks similar to those of other early-stage immuno-oncology companies, including dependence on key individuals; the need to develop commercially viable therapeutics; competition from other companies, many of which are larger and better capitalized; and the need to obtain adequate additional financing to fund the development of its products. The Company currently depends on third-party suppliers for key materials and services used in its research and development manufacturing process, and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate materials and services.

Cash, Cash Equivalents, and Restricted Cash

Cash equivalents, which consist primarily of highly liquid investments with maturities of three months or less when purchased, are stated at cost which approximates fair value. These assets include investments in money market funds that invest in U.S. Treasury obligations and certificates of deposit which are stated at fair value.

The Company has issued a letter of credit under a lease agreement which has been collateralized by a cash deposit for an equal amount and is recorded within deposits and other long-term assets on the balance sheet based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows (in thousands).

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	34,572	$	52,183
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	35,564	$	53,175

Leases

Prior to January 1, 2019, the Company rented its office space and facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. The Company’s lease agreements contained rent holidays, scheduled rent increases, and renewal options. Rent holidays and scheduled rent increases were included in the determination of rent expense to be recorded ratably over the lease term. The Company did not assume renewals in its determination of the lease term unless they were deemed to be reasonably assured at the inception of the lease. The Company began recognizing rent expense on the date that it obtained the legal right to use and control the leased space. Deferred rent consisted of the difference between cash payments and the recognition of rent expense on a straight-line basis for the buildings the Company occupied.

Funding of leasehold improvements by the Company’s landlord was accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.

In certain arrangements, the Company was involved in the construction of improvements to buildings it was leasing. To the extent the Company was involved with the structural improvements of the construction project or takes construction risk, the Company was considered to be the owner of the building and related improvements for accounting purposes during the construction period. The Company recorded the fair value of the building and related improvements subject to the lease within property and equipment on the balance sheet. The Company also recorded a corresponding lease financing obligation on its balance sheet representing the amounts financed by the lessor for the building and lessor financed improvements. Lessor financed improvement incentives due but not yet received of $1.2 million at December 31, 2017 were recorded as prepaid expense and other current assets on the condensed balance sheet. Such amounts were fully collected in April 2018. Once a construction project was complete, the Company considered the requirements for sale-leaseback accounting treatment. If the Company concluded the arrangement did not qualify for sale-leaseback accounting treatment, the building and related improvements remained on the Company’s condensed balance sheet and were subject to depreciation and assessment of impairment.

For such arrangements, at both pre and post the construction period, the Company bifurcated its lease payments into a portion allocated to the building and a portion allocated to the parcel of land on which the building had been built. The portion of the lease payments allocated to the land was treated for accounting purposes as operating lease payments, and therefore was recorded as rent expense in the condensed statements of operations and comprehensive loss. The portion of the lease payments allocated to the building were further bifurcated into a portion allocated to interest expense and a portion allocated to reduce the lease financing obligation. The interest rate used for the lease financing obligation represented the Company’s estimated incremental borrowing rate at the inception of the lease, adjusted to reduce any built in loss.

Subsequent to January 1, 2019, the Company determines whether the arrangement is or contains a lease at the inception of the arrangement and if such a lease is classified as a financing lease or operating lease. All of the Company’s leases are classified as operating leases. Leases with a term greater than one year are included in operating lease right-of-use assets, lease liabilities, current portion, and lease liabilities, net of current portion in our condensed balance sheet at March 31, 2019. The Company has elected not to recognize on the condensed balance sheet leases with terms of one year or less. Lease liabilities and their corresponding right-of-use assets are recorded based on the present value of lease payments over the expected lease term. In determining the net present value of lease payments, the interest rate implicit in lease contracts is typically not readily determinable. As such, the Company utilizes the appropriate incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment. Certain adjustments to the right-of-use asset may be required for items such as initial direct costs paid or incentives received and impairment charges if we determine the ROU Asset is impaired.

The Company considers a lease term to be the noncancelable period that it has the right to use the underlying asset, including any periods where it is reasonably assured the Company will exercise the option to extend the contract. Periods covered by an option to extend are included in the lease term if the lessor controls the exercise of that option.

The Company recognizes lease expense on a straight-line basis over the expected lease term.

The Company has elected to not separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. The lease components resulting in a right-of-use asset have been recorded on the condensed balance sheet and amortized as lease expense on a straight-line basis over the lease term.

Revenue Recognition

The Company analyzes its collaboration agreements to assess whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities. This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For collaboration arrangements that are considered to be in the scope of the collaboration guidance and that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of the collaboration guidance and those that are more reflective of a vendor-customer relationship and, therefore, within the scope of the revenue with contracts with customer guidance. For elements of collaboration arrangements that are accounted for pursuant to the revenue from contracts with customer guidance, an appropriate recognition method is determined and applied consistently, generally by analogy to the revenue from contracts with customers guidance.

The terms of the licensing and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front fees; development, regulatory, and commercial milestone payments; payments for manufacturing supply services; and royalties on net sales of licensed products. Each of these payments results in license, collaboration, and other revenues, except for revenues from royalties on net sales of licensed products, which are classified as royalty revenues. The core principle of the accounting for revenue from contracts with customers guidance is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received in exchange for those goods or services.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue in the Company’s balance sheets. If the related performance obligation is expected to be satisfied within the next twelve months this will be classified in current liabilities. Amounts recognized as revenue prior to receipt are recorded as contract assets in the Company’s balance sheets. If the Company expects to have an unconditional right to receive consideration in the next twelve months, this will be classified in current assets. A net contract asset or liability is presented for each contract with a customer.

At contract inception, the Company assesses the goods or services promised in a contract with a customer and identifies those distinct goods and services that represent a performance obligation. A promised good or service may not be identified as a performance obligation if it is immaterial in the context of the contract with the customer, if it is not separately identifiable from other promises in the contract (either because it is not capable of being separated or because it is not separable in the context of the contract), or if the performance obligation does not provide the customer with a material right.

The Company considers the terms of the contract and its customary business practices to determine the transaction price. The transaction price is the amount of consideration to which the Company expects to be entitled in exchange for transferring promised goods or services to a customer. The consideration promised in a contract with a customer may include fixed amounts, variable amounts, or both. Variable consideration will only be included in the transaction price when it is not considered constrained, which is when it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur.

If it is determined that multiple performance obligations exist, the transaction price is allocated at the inception of the agreement to all identified performance obligations based on the relative standalone selling prices. The relative selling price for each deliverable is estimated using objective evidence if it is available. If objective evidence is not available, the Company uses its best estimate of the selling price for the deliverable.

Revenue is recognized when, or as, the Company satisfies a performance obligation by transferring a promised good or service to a customer. An asset is transferred when, or as, the customer obtains control of that asset, which for a service, is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the good or service promised to the customer.

After contract inception, the transaction price is reassessed at every period end and updated for changes such as resolution of uncertain events. Any change in the transaction price is allocated to the performance obligations on the same basis as at contract inception.

Management may be required to exercise considerable judgment in estimating revenue to be recognized. Judgment is required in identifying performance obligations, estimating the transaction price, estimating the stand-alone selling prices of identified performance obligations, which may include forecasted revenue, development timelines, reimbursement rates for personnel costs, discount rates and probabilities of technical and regulatory success, and estimating the progress towards satisfaction of performance obligations.

Recently Issued Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (“FASB”) issued ASU 2016-13, Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments (“ASU No. 2016-13”), which requires that expected credit losses relating to financial assets measured on an amortized cost basis and available-for-sale debt securities be recorded through an allowance for credit losses. ASU No. 2016-13 limits the amount of credit losses to be recognized for available-for-sale debt securities to the amount by which carrying value exceeds fair value and also requires the reversal of previously recognized credit losses if fair value increases. The new standard will be effective on January 1, 2020. Early adoption is permitted. The Company is currently evaluating the effect that the updated standard will have on its financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU No. 2018-13 eliminates, adds and modifies certain disclosure requirements for fair value measurements and requires companies to disclose certain information. The new standard will be effective for fiscal years, and interim periods within those year, beginning after December 15, 2019. The Company is currently evaluating the impact of adopting this accounting update on its financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract, which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for annual reporting periods beginning after December 15, 2019, and interim periods within that year. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact of adoption on its financial statements.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 , which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The standard adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. The standard is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. Amendments in the standard should be applied retrospectively to the date of initial application of Topic 606, but entities may elect to apply the amendments in this Update retrospectively either to all contracts or only to contracts that are not completed at the date of initial application of Topic 606, and should disclose the election. An entity may also elect to apply the practical expedient for contract modifications that is permitted for entities using the modified retrospective transition method in Topic 606. The Company is currently assessing the impact of this standard on its condensed financial statements.

Recently Adopted Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). The new standard requires the recognition of lease liabilities and right-of-use (ROU) assets on the balance sheet arising from lease transactions at the lease commencement date and the disclosure of key information about leasing arrangements. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842) Targeted Improvements, which provides an additional transition method in which the new lease standard is applied at the adoption date and recognized as a cumulative-effect adjustment to retained earnings without adjustment to comparative periods (collectively “Topic 842”). The amendment has the same effective date and transition requirements as the new lease standard.

The Company adopted this standard on January 1, 2019 using the modified retrospective approach and elected the package of practical expedients permitted under transition guidance, which allowed the Company to carry forward its historical assessments of: 1) whether contracts are or contain leases, 2) lease classification and 3) initial direct costs, where applicable. The Company did not elect the practical expedient allowing the use-of-hindsight which would require the Company to reassess the lease term of its leases based on all facts and circumstances through the effective date and did not elect the practical expedient pertaining to land easements as this is not applicable to the current contract portfolio. The Company elected the post-transition practical expedient to not separate lease components from non-lease components for all existing lease classes. The Company also elected a policy of not recording leases on its condensed balance sheets when the leases have a term of 12 months or less and the Company is not reasonably certain to elect an option to purchase the leased asset.

The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows (in thousands):

	December 31, 2018			Adjustments due to Adoption of Topic 842			January 1, 2019
Property and equipment, net	$	29,494		$	(14,524	)	$	14,970
Operating lease right-of-use assets	$	-		$	14,224		$	14,224
Operating liabilities:
Lease liabilities, current portion	$	-		$	2,200		$	2,200
Accrued liabilities	$	992		$	(475	)	$	517
Deferred rent, net of current portion	$	1,353		$	(1,353	)	$	-
Lease financing obligation, net of current portion	$	10,490		$	(10,490	)	$	-
Lease liabilities, net of current portion	$	-		$	8,980		$	8,980
Accumulated deficit	$	(126,402	)	$	(153	)	$	(126,555	)

The adjustments due to the adoption of Topic 842 primarily related to the recognition of operating lease ROU Assets and lease liabilities for the Company’s operating leases. In addition, the adoption of Topic 842 resulted in a change in classification of build-to-suit component of our lease in Pleasanton, California to an operating lease and resulted in the derecognition of the $15.4 million capitalized building and related accumulated depreciation of $0.9 and $10.5 million financing lease obligation, as the Company had been deemed to own the building under legacy GAAP (Note 6). The Company also recorded an insignificant reduction to opening accumulated deficit as of January 1, 2019 as a result of the adoption of Topic 842.

The impact of the adoption of Topic 842 on the accompanying condensed statements of operations was not material.

In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. These amendments eliminate, modify, or integrate into other SEC requirements certain disclosure rules. Among the amendments is the requirement to present an analysis of changes in stockholders’ equity in the interim financial statements included in quarterly reports on Form 10-Q. The analysis, which can be presented as a footnote or separate statement, is required for the current and comparative quarter and year-to-date interim periods. The amendments are effective for all filings made on or after November 5, 2018. In light of the anticipated timing of effectiveness of the amendments and expected proximity of effectiveness to the filing date for most filers’ quarterly reports, the SEC’s Division of Corporate Finance issued a Compliance and Disclosure Interpretation related to Exchange Act Forms, or CDI – Question 105.09, that provides transition guidance related to this disclosure requirement. CDI – Question 105.09 states that the SEC would not object if the filer’s first presentation of the changes in shareholders’ equity is included in its Form 10-Q for the quarter that begins after the effective date of the amendments. As such, the Company adopted these SEC amendments on November 5, 2018 and has presented the analysis of changes in stockholders’ equity in its interim financial statements in this Form 10-Q for the quarter ending March 31, 2019. Adoption of these SEC amendments did not have a material effect on the Company’s financial position, results of operations, cash flows or stockholders’ equity.

In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU No. 2018-07”). ASU No. 2018-07 is intended to reduce cost and complexity and to improve financial reporting for share-based payments issued to non-employees (for example, service providers, external legal counsel, suppliers, etc.). The ASU expands the scope of Topic 718, Compensation—Stock Compensation, which currently only includes share-based payments issued to employees, to also include share-based payments issued to non-employees for goods and services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU No. 2018-07 is effective for annual and interim periods beginning after December 15, 2018. Early adoption of the standard is permitted. The standard will be applied in a retrospective approach for each period presented. The Company adopted the standard during the quarter ended March 31, 2019, which did not have a material impact on its financial statements and related disclosures.

Cash Equivalents and Marketable Securities

3 Months Ended

Mar. 31, 2019

Cash And Cash Equivalents [Abstract]

Cash Equivalents and Marketable Securities

3.	Cash Equivalents and Marketable Securities

The amortized cost, unrealized gains and losses and fair values of cash equivalents and marketable securities were as follows (in thousands):

	March 31, 2019
Description	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Money market funds	$	28,813	$	—	$	—	$	28,813
Commercial paper		55,212		48		—		55,260
Corporate debt securities		43,310		20		—		43,330
	$	127,335	$	68	$	—	$	127,403
Classified as:
Cash equivalents							$	29,813
Marketable securities								97,590
Total							$	127,403

	December 31, 2018
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	36,148	$	—	$	—		$	36,148
Commercial paper		45,244		—		(40	)		45,204
Corporate debt securities		67,815		1		(46	)		67,770
	$	149,207	$	1	$	(86	)	$	149,122
Classified as:
Cash equivalents								$	48,195
Marketable securities									100,927
Total								$	149,122

All marketable securities held as of March 31, 2019, had contractual maturities of less than one year. There have been no realized gains or losses on marketable securities for the periods presented. None of the Company’s investments in marketable securities has been in an unrealized loss position for more than one year. The Company determined that it has the ability and intent to hold all marketable securities that have been in a continuous loss position until maturity or recovery, thus there has been no recognition of any other-than-temporary impairment in the three months ended March 31, 2019 and 2018.

See Note 4 for further information regarding the fair value of our financial instruments.

Fair Value Measurements

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

4.	Fair Value Measurements

The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritizes the inputs into three broad levels as follows:

•

Level 1 inputs are quoted prices in active markets that are accessible at the market date for identical assets or liabilities.

•

Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly.

•

Level 3 inputs are unobservable inputs that reflect the Company’s own assumptions about the assumptions market participants would use in pricing the assets or liability. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement.

The carrying amounts reflected on the balance sheets for cash and cash equivalents, prepaid expenses and other current assets, accounts payable, accrued compensation and accrued liabilities approximate their fair values due to their short-term nature.

The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	March 31, 2019
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	28,813	$	28,813	$	—	$	—
Commercial paper		55,260		—		55,260		—
Corporate debt securities		43,330		—		43,330		—
Total	$	127,403	$	28,813	$	98,590	$	—

	December 31, 2018
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	36,148	$	36,148	$	—	$	—
Commercial paper		45,204		—		45,204		—
Corporate debt securities		67,770		—		67,770		—
Total	$	149,122	$	36,148	$	112,974	$	—

The Company measures the fair value of money market funds based on quoted prices in active markets for identical securities. Commercial paper and corporate debt securities are valued taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs.

There were no transfers between Level 1 and Level 2 during the periods presented. See Note 3 for further information regarding the amortized cost of our financial instruments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Commitments and Contingencies

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6.	Commitments and Contingencies

Leases

In November 2015, the Company entered into an 84-month non-cancelable operating lease, effective March 2016, for a facility in Emeryville, California, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $50,000. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional three years at the prevailing rental rate. In September 2018 the Emeryville lease was amended whereby the Company entered into a 12-month operating lease for additional temporary space. The Company may terminate the temporary space lease agreement with 30 days advanced written notice to the landlord.

In January 2019, the Company entered into a 120-month operating lease for a new facility in Emeryville, California with office and laboratory space for the Company’s new principal executive offices. In conjunction with signing the lease, the Company paid a cash security deposit of $0.6 million, which is recorded as a deposit on the Company’s condensed balance sheet as of March 31, 2019. The lease agreement includes a free rent period, an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional two five-year periods at the then market rental rate. The lessor provided the Company a tenant improvement allowance for a total of $4.0 million to complete the laboratory and office renovation. The Company’s obligation to pay rent will commence on November 1, 2019. The Company has determined the tenant improvements to be lessee owned and therefore has recorded a $10.4 million ROU Asset and a $10.7 million lease liability on the condensed balance sheet as of March 31, 2019.

In connection with the new lease agreement, the Company also entered into an agreement (the “Lease Termination Agreement”) to early terminate the Company’s existing lease dated November 2015, for its current premises. The current lease will terminate effective no later than 60 days after the rent commencement date under the new lease, which in October 2019. The Company accounted for the Lease Termination Agreement as a separate contract and recorded an adjustment of $1.8 million, which is included within the March 31, 2019 condensed balance sheet, to the ROU Asset and lease liability to reflect the remaining term of the modified agreement through October 2019.

In February 2016, the Company entered into a 67-month non-cancelable operating lease effective October 2016 for a facility in Cambridge, Massachusetts, with laboratory and office space. In conjunction with signing the lease, the Company paid a cash security deposit of $0.3 million, which is recorded in deposits and other long term assets on the Company’s condensed balance sheet as of March 31, 2019. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional three years at the prevailing rental rate. The lessor provided the Company a tenant improvement allowance for a total of $2.1 million to complete the laboratory and office renovation. The Company recorded the tenant allowance received as leasehold improvements under the property and equipment account and deferred rent liability on the accompanying condensed balance sheets. Upon adoption of Topic 842, the deferred rent liability was reclassified against the ROU Asset on the condensed balance sheet as of January 1, 2019.

In March 2017, the Company entered into a noncancelable operating lease (the “Pleasanton Lease”) to lease 42,620 square feet of office, cleanroom, and laboratory support manufacturing space in Pleasanton, California (the “Pleasanton Facility”). Subsequently, in April 2017, the Company took possession of the space. The Pleasanton Lease includes a free rent period, escalating rent payments and a term that expires on November 30, 2024. The Company has the option to extend the lease term for a period of five years at the then market rental rate. The Company’s obligation to pay rent commenced in December 2017. The Company obtained an irrevocable letter of credit in March 2017 in the initial amount of approximately $1.0 million as a security deposit to the Pleasanton Lease, which may be drawn down by the landlord in the event the Company fails to fully and faithfully perform all of its obligations. The letter of credit may be reduced based on certain levels of cash and cash equivalents the Company holds. The Pleasanton Lease further provides that the Company is obligated to pay to the landlord its proportionate share of certain basic operating costs, including taxes and operating expenses.

In connection with the Pleasanton Lease, the Company received a tenant improvement allowance of $1.2 million from the landlord for the costs associated with the design, development and construction of tenant improvements for the Pleasanton Facility. The scope of the tenant improvements did not qualify under the lease accounting guidance as “normal tenant improvements” and the Company was deemed owner of the leased building during the construction period for accounting purposes. The Company had therefore capitalized the $9.3 million fair value of the leased building within property and equipment, net, and recognized a corresponding non-current lease financing obligation in the condensed balance sheet as of December 31, 2018. The fair value of the leased building was estimated using a market approach that utilized comparable observable sales for similar assets (Level 2 inputs). The Company had also recognized building improvements totaling $6.1 million for additions to the leased building incurred by the Company during the construction period, of which $1.2 million were due but had not yet been received from the landlord as of December 31, 2017 and were recorded as an increase to the lease financing obligation and prepaid and other current assets on the condensed balance sheet at that time. Such amounts were subsequently reimbursed by the landlord in April 2018. In November 2017, construction on the Pleasanton Facility was substantially completed and the leased property was placed into service. The Company determined the completed construction project did not qualify for sale-leaseback accounting due to the collateral held by the landlord in the form of a letter of credit and instead was accounted for as a financing lease transaction. The leased building for the Pleasanton Facility and related improvements remained on the Company’s balance sheet as of December 31, 2018 and rental payments associated with the Pleasanton Lease were allocated to operating lease expense for the ground underlying the leased building and principal and interest payments on the lease financing obligation.

Upon adoption of Topic 842, the Company analyzed the Pleasanton lease under the new guidance and determined that the lease would be classified as an operating lease under legacy GAAP. Additionally, given the Company had previously recognized the building and financing lease obligation solely as a result of the transactions build to suit designation under legacy GAAP, the Company derecognized the $14.5 million leased building and $10.5 million lease financing obligation from the condensed balance sheet on January 1, 2019. The unamortized tenant improvement allowance of $4.0 million and was recognized as a component of ROU Assets on January 1, 2019. The Company also recorded a $0.2 million reduction to opening accumulated deficit as of January 1, 2019.

In September 2018, the Company entered into a 24-month non-cancellable operating lease for an additional facility in Cambridge, Massachusetts with laboratory and office space. In conjunction with signing the lease, the Company prepaid the first twelve months base rent in the amount of $1.3 million, of which the remaining amount of $0.9 million as of January 1, 2019 was reclassified to the ROU Asset on the condensed balance sheet upon adoption of Topic 842. The Company also paid a cash security deposit of $0.3 million, which included $0.1 million for the last month’s rent and was reclassified to the ROU Assets on the condensed balance sheet upon adoption of Topic 842 on January 1, 2019. The remaining security deposit is recorded in deposits and other long term assets on the Company’s condensed balance sheet as of March 31, 2019.

The Company’s operating leases include various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature.

The components of lease costs, which were included in our condensed statements of operations and comprehensive loss, were as follows (in thousands):

	Three Months Ended March 31,
	2019
Lease cost
Operating lease cost	$	1,344
Short-term lease cost		56
Total lease cost	$	1,400

Supplemental information related to leases was as follows (in thousands):

	Three Months Ended March 31,
	2019
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$	640
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$	10,542
Weighted average remaining lease term (years):
Operating leases		7.6
Weighted average discount rate:
Operating leases		9.0	%

As of March 31, 2019, minimum annual rental payments under the Company’s operating lease agreements are as follows (in thousands):

	Operating Leases
Year ending December 31:
2019 (remaining nine months)	$	2,386
2020		5,058
2021		4,267
2022		3,623
2023		3,337
Thereafter		15,876
Total minimum payments	$	34,547
Less: Amounts representing interest		(10,836	)
Less: Amounts representing tenant improvement allowance		(4,033	)
Present value of future minimum lease payments		19,678
Less: Current portion of lease liability		(2,574	)
Noncurrent portion of lease liability	$	17,104

The amount representing the tenant improvement allowance is expected to be received by the Company concurrent with the completion of construction in late 2019.

Agreement with CRO

In September 2017, the Company entered into a contract research and development agreement with a third party contract research organization (“CRO”) to provide research, analysis and antibody samples to further the Company’s development of its drug candidates. Under the agreement, the Company paid an upfront payment of $0.5 million to the CRO. The upfront payment was capitalized and recognized as research and development expense using the straight-line method over the term of the agreement, which is one year and ended on December 31, 2018. During the year ended December 31, 2018, the Company recognized a total of $1.1 million of research and development expense under the agreement. The Company is also obligated to pay the CRO certain milestone payments of up to $36.4 million on achievement of specified events. None of these events had occurred as of March 31, 2019. During the three months ended March 31, 2019, the Company recognized an insignificant amount of research and development expense under the agreement. During the three months ended March 31, 2018, the Company recognized a total of $0.3 million of research and development expense under the agreement.

Guarantees and Indemnifications

The Company, as permitted under Delaware law and in accordance with its certification of incorporation and bylaws, and pursuant to indemnification agreements with certain of its officers and directors, indemnifies its officers and directors for certain events or occurrences, subject to certain limits, which the officer or director is or was serving at the Company’s request in such capacity. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance limits the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Collaboration and License Agreements

3 Months Ended

Mar. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaboration and License Agreements

8.	Collaboration and License Agreements

bluebird bio, Inc.

In August 2018, the Company entered into a Research Collaboration and License Agreement (“Collaboration Agreement”) with bluebird bio, Inc. (“bluebird”). Under the terms of the Collaboration Agreement, the Company will provide to bluebird tumor-specific targets across several tumor types and, in certain cases, T-cell receptors (“TCR”) directed to those targets. The Company received a non-refundable upfront payment of $20.0 million and bluebird also concurrently acquired 768,115 shares of the Company’s Series C convertible preferred stock for $10.0 million at $13.04 per share. Per the Collaboration Agreement, bluebird was also provided an option to acquire shares of the Company’s common stock at the same price as all other investors in connection with the IPO. In October 2018, bluebird purchased 666,667 shares of the Company’s common stock at the price to the public of $15.00 per share for a total of $10.0 million. Under the terms of the Collaboration Agreement, the Company is eligible to earn development, regulatory, and sales-based milestones in an amount of up to $1.2 billion, and single-digit royalties on sales of products that utilize the technology subject to the Collaboration Agreement. None of these events had occurred as of March 31, 2019 and no royalties were due from the sale of licensed products.

bluebird may terminate the Collaboration Agreement by giving a 120 day prior written notice to the Company at any time after the effective date of the agreement. Unless terminated early the agreement has a term that ends upon the last payment owed by Gritstone on a licensed product. The Collaboration Agreement may be terminated for cause by either party based on uncured material breach by the other party or bankruptcy of the other party. Upon early termination, all ongoing activities under the agreement and all mutual collaboration, development and commercialization licenses and sublicenses will terminate. The licenses granted by the Company to bluebird under the licensed intellectual property will remain in effect in accordance with their respective terms. Additionally, all of bluebird’s payment obligations that have not yet accrued related to future milestone and royalty payments will be reduced by fifty percent for the remainder of the agreement term.

The Company concluded that bluebird is a customer, and the contract is not subject to accounting literature on collaborative arrangements. This is because the Company granted to bluebird a license to its intellectual property, and provided research and development services, all of which are outputs of the Company’s ongoing activities, in exchange for consideration.

The Company identified the following three material promises under the Collaboration Agreement: 1) transfer of a license to intellectual property and related technology know-how (“License and Know-How”); 2) the obligation to perform target selection and TCR generation services (“Research and Development Services”); and 3) participation on the Joint Steering Committee (“JSC”). The Company provided to bluebird standard indemnification and protection of licensed intellectual property, which is part of assurance that the license meets the contract’s specifications and is not an obligation to provide goods or services.

The Company considered that the License and Know-How has standalone functionality, was considered to be functional intellectual property, and is capable of being distinct. However, the Company determined that the License and Know-How is not distinct from the Research and Development Services or participation on the JSC within the context of the agreement because bluebird is dependent on the Company to execute the Research and Development Services and participate on the JSC in order for bluebird to benefit from the License and Know-How. As such, the License and Know-How is combined with the Research and Development Services and participation on the JSC into a single performance obligation. As such, the transaction price under this arrangement will be allocated to this single performance obligation.

The Company has also determined that all other goods or services which are contingent upon bluebird reaching various milestones are not considered performance obligations at the inception of the arrangement.

The transaction price at the inception of the Collaboration Agreement consisted of the upfront payment of $20.0 million and the $10.0 million received from bluebird for the purchase of the Company’s Series C convertible preferred stock. The sale of the Series C convertible preferred stock was not considered to be a performance obligation as it was a separate financing component of the transaction. Accordingly, $10.0 million of the transaction price was allocated to the issuance of 768,115 shares of Series C convertible preferred stock at fair value of $13.04 per share and recorded in stockholders’ equity.

The variable consideration related to the remaining development, regulatory, and sales-based milestones payments has not been included in the initial transaction price and continues to be fully constrained as of March 31, 2019. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestones is outside the control of the Company and contingent upon initiation of clinical trials for early stage targets and bluebird’s development efforts. Any variable consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as they were determined to relate predominantly to the License and Know-How granted to bluebird. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur.

For revenue recognition purposes, the Company determined that the duration of the contract began on the effective date in August 2018 and ends upon completion of the Research and Development Services which is also when the participation on the JSC is no longer an obligation. The contract duration is defined as the period in which parties to the contract have present enforceable rights and obligations. We analyzed the impact of bluebird terminating the agreement prior to August 2023 and determined that there were substantive non-monetary penalties to bluebird for doing so. We considered quantitative and qualitative factors to reach this conclusion.

Revenue is recognized when, or as, the Company satisfies its performance obligation by transferring the promised services to bluebird. Revenue will be recognized over time using a cost-based input method, based on internal labor cost effort to perform the research services, since the internal labor cost incurred over time is thought to best reflect the transfer of services to bluebird. In applying a cost-based input method of revenue recognition, we use actual costs incurred relative to budgeted costs to fulfill the combined performance obligation. A cost-based input method of revenue recognition requires us to make estimates of costs to complete the performance obligation. The cumulative effect of any revisions to estimated costs to complete the performance obligation will be recorded in the period in which changes are identified and amounts can be reasonably estimated. A significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods.

For the three months ended March 31, 2019, the Company has recognized $1.3 million as collaboration revenue as a result of satisfying its performance obligation by transferring the promised services estimated by the labor cost incurred. A deferred revenue balance of $17.5 million is recorded on the condensed balance sheet in both current and long-term liabilities as of March 31, 2019, which relates to the performance obligation identified, with such amounts to be recognized over the period the performance obligation is expected to be satisfied, which is currently expected to be through mid-2023.

Changes in the deferred revenue balance during the three months ended March 31, 2019 are as follows (in thousands):

	Deferred Revenue
Balance at January 1, 2019	$	18,813
Additions		—
Deductions		(1,347	)
Balance at March 31, 2019	$	17,466

There were no receivables or net contract assets recorded as of March 31, 2019 associated with the Collaboration Agreement.

The Company expensed all incremental costs of obtaining Collaboration Agreement as such amounts were insignificant.

Arbutus Biopharma Corporation

In October 2017, the Company entered into an Exclusive License Agreement by and between Arbutus Biopharma Corporation (“Arbutus”) and Protiva Biotherapeutics Inc. a wholly owned subsidiary of Arbutus. Certain terms of the agreement were modified by amendment in July 2018. Under the license agreement, the Company has an exclusive license to utilize certain Arbutus intellectual property including patents and know-how relating to immunotherapy. Under this license agreement, the Company paid an upfront payment of $5.0 million which was included in research and development expenses during 2017. The Company also reimbursed Arbutus for materials and personnel costs totaling $0.2 million, which were included in research and development expenses during 2017. During the three months ended March 31, 2019 and 2018, the Company reimbursed Arbutus for an insignificant amount for materials and personnel costs. The Company is obligated to pay Arbutus certain milestone payments up to $123.5 million on achievement of specified events, and royalties of not more than 3.5% on sales of its licensed products. Following the acceptance of our investigational new drug application for GRANITE-001 by the U.S. Food and Drug Administration, the Company made a $2.5 million development milestone payment to Arbutus in September 2018 that was recorded as research and development expense. None of the other events had occurred as of March 31, 2019 and no royalties were due from the sale of licensed products.

Non-Profit Hospital Cancer Center

In January 2016, the Company entered into an Exclusive License Agreement with a non-profit hospital cancer center. Under the license agreement, the Company has an exclusive license to utilize certain patents and know-how relating to immunotherapy for an insignificant upfront payment, cash milestone payments on achievement of specified events, and a low single digit royalty on sales of licensed products. The achievement of the milestones and payment of royalties is dependent upon obtaining regulatory approval. Upon achievement of a milestone related to the Company’s Phase 1 clinical trial, in December 2018 the Company recorded $50,000 to research and development expense for amounts owed to the Hospital Cancer Center, which was paid to the hospital in February 2019. None of the other milestone events had occurred as of March 31, 2019 and no royalties were due from the sales of licensed products. The Company also issued a ten-year warrant to the cancer center for the right to purchase 40,257 shares of its common stock at $0.35 per share. The estimated fair value of the warrant was not significant and was included in research and development expense and additional paid-in-capital. The warrant was exercised in full in January 2018.

Convertible Preferred Stock and Common Stock

3 Months Ended

Mar. 31, 2019

Equity [Abstract]

Convertible Preferred Stock and Common Stock

9.	Convertible Preferred Stock and Common Stock

Convertible Preferred Stock

The Company’s Series A, Series B and Series C convertible preferred stock has various features, including convertibility and non-cumulative dividends. The Company determined that none of the features required bifurcation from the underlying shares, either because they are clearly and closely related to the underlying shares or because they do not meet the definition of a derivative. The Series A, Series B and Series C convertible preferred stock are considered permanent equity and have not been accreted up to their redemption value. The Second and Third Tranche rights are considered to be mutual options as neither the purchasers nor the Company have a commitment or obligation to purchase or sell additional shares. As such, these rights are not accounted for separately. Moreover, in any such redemption (i.e. deemed liquidation) all equity holders (common and preferred) will receive the same form of consideration. The preferred stockholders cannot contractually redeem their shares, or redeem their shares through separate negotiation, without the Company’s common stockholders being able to also redeem their shares for the same form of consideration.

The Company entered into a Series C preferred stock purchase agreement (“Series C Preferred Stock Purchase Agreement”), with certain investors in June 2018, and upon approval by the Company’s Board of Directors, the Company completed a Series C convertible preferred stock financing (“Series C”) at a price per share of $13.04. The net cash proceeds totaled $8.9 million and 690,128 shares of Series C convertible preferred stock were issued. Issuance costs totaled $0.1 million and were recorded as a reduction of the proceeds. In July 2018, the Company sold an additional 153,360 shares of Series C convertible preferred stock at a price of $13.04 per share for net cash proceeds of $2.0 million. In August 2018, in conjunction with the Collaboration Agreement entered into with bluebird, the Company sold bluebird 768,115 shares of Series C convertible preferred stock at a price of $13.04 per share for gross proceeds of $10.0 million.

In October 2018, upon closing of the IPO, all outstanding shares of convertible preferred stock converted into 19,409,132 shares of common stock. Per the terms of the Convertible Preferred Stock Agreement, the shares of outstanding convertible preferred stock converted at the election of the holders of at least a majority of the then outstanding shares of convertible preferred stock, voting together as a single class and on an as-converted to common stock basis.

At March 31, 2019 and December 31, 2018, there were 10,000,000 shares of convertible preferred stock authorized and no shares were outstanding.

Prior to the conversion of the convertible preferred stock upon closing of the IPO in October 2018, the rights, preferences, and privileges of the convertible preferred stock were as follows:

Redemption Rights

The preferred stock is not redeemable by holders unless a redemption event occurs. A redemption event will only occur upon the liquidation or winding up of the Company, a greater than 50% change in control, or the sale of substantially all of the assets of the Company. Management has also elected not to adjust the carrying values of the Series A, Series B and Series C convertible preferred stock to the redemption value of such shares, since it is uncertain whether or when a redemption event will occur. Subsequent adjustments to increase the carrying value to the redemption values will be made when it becomes probable that such a redemption will occur.

Dividends Rights

The holders of Series A, Series B and Series C convertible preferred stock are entitled to receive dividends, from any assets legally available, prior and in preference to any declaration or payment of any dividend to the common stockholders, at the rate of 8% of the original issue price (as determined on a per annum basis and on an as-converted basis). Such dividends are payable if and when declared by the Board of Directors and are not cumulative. After payment of such dividends, any additional dividends shall be distributed among the holders of the Series A, Series B and Series C convertible preferred stock and common stock pro rata based on the number of shares of common stock then held by each holder (assuming conversion of all such preferred stock into common stock). No such dividends were ever declared or accrued.

Liquidation Rights

In the event of any liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary (“Liquidation Event”), the holders of Series C convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of the Series A and B convertible preferred stock, $13.04 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends). The holders of Series B convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of the Series A convertible preferred stock, $10.76 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends). After payment of the above, the holders of Series A convertible preferred stock are entitled to receive, prior and in preference to any distribution of any of the assets of the Company to the holders of common stock, $6.90 per share (as adjusted for any stock splits, combinations, reorganizations, or similar transactions, plus any declared and unpaid dividends).

If, upon the occurrence of such an event, the proceeds to be distributed are insufficient to permit the payment to such holders of the full preferential amounts, then the entire amount legally available for distribution shall be distributed among the holders of the Series A, Series B and Series C preferred stock in proportion to the full preferential amount that each such holder is otherwise entitled to receive had such proceeds been available.

After liquidation preference payments have been made to the holders of the convertible preferred stock as described above, all of the remaining assets and funds of the Company are to be distributed ratably among the holders of the preferred and common stock, as if the preferred stock had been converted to common stock. However, Series C preferred stock holders are limited to the greater of (1) $65.21 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series C convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company. Series B preferred stockholders are limited to the greater of (1) $53.82 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series B convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company. Series A preferred stockholders are limited to the greater of (1) $34.50 per share (as adjusted for any stock splits, combinations, reorganizations or similar transactions) and (2) the amount the holder would have received if all shares of Series A convertible preferred stock had been converted to common stock prior to such liquidation, dissolution, or winding up of the Company.

Voting Rights

Except as otherwise required by law, the holders of common and Series A, Series B and Series C convertible preferred stock vote together as a single class. The holders of the convertible preferred stock are entitled to the number of votes equal to the number of shares of common stock into which the convertible preferred stock could be converted on the record date for the vote, or upon the written consent of the stockholders.

The holders of the Series A convertible preferred stock are entitled to elect three directors of the Company, the holders of the Series B convertible preferred stock are entitled to elect one director of the Company, and the holders of common stock shall be entitled to elect one director of the Company.

Conversion Rights

Each share of Series A, Series B and Series C convertible preferred stock, at the option of the holder and at any time after the date of issuance, is convertible into the number of shares of common stock determined by dividing the respective original issue price by the conversion price (the Conversion Price). The Company’s Series A, Series B, and Series C Conversion Prices were $6.90, $10.76, and $13.04, respectively, and were subject to certain future adjustments.

As part of the Company’s Series C convertible preferred stock financing, the Company’s certificate of incorporation was amended to reduce the public offering automatic conversion price for the Series A and B convertible preferred stock from $21.53 to $15.66 per share. The Company accounted for this as a modification of an instrument akin to equity that resulted in no incremental fair value being attributed to the Series A and Series B convertible preferred stock.

Common Stock

The Company is authorized to issue 300,000,000 shares of common stock. Holders of common stock are generally entitled to one vote per share on all matters to be voted upon by the stockholders of the Company.

Subject to the preferences that may be applicable to any outstanding shares of preferred stock, the holders of common stock are entitled to receive ratably such dividends, if any, as may be declared by the Board of Directors. No dividends have been declared to date.

Stock-Based Compensation

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

10.	Stock-Based Compensation

Award Incentive Plans

In August 2015, the Board of Directors approved the 2015 Equity Incentive Plan (“2015 Plan”). In February 2018, the Company’s Board of Directors approved a 507,246 share increase in the number of shares to be reserved under the Company’s 2015 Equity Incentive Plan. In connection with the Company’s IPO and the effectiveness of the 2018 Award Incentive Plan (“2018 Plan”), the 2015 Plan terminated and no further awards will be granted under the 2015 Plan. The 92,815 shares of common stock shares that were then unissued and available for future issuance under the 2015 Plan became available under the 2018 Plan. The 2015 Plan will continue to govern all outstanding awards by their existing terms.

In September 2018, the Company’s Board of Directors approved the 2018 Plan. Under the 2018 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other certain awards to individuals who are employees, officers, directors or consultants of the Company. A total of 2,690,000 shares of our common stock are initially reserved for issuance pursuant to a variety of stock-based compensation awards, including stock options, stock appreciation rights, or SARs, restricted stock awards, restricted stock unit awards and other stock-based awards, plus the number of shares remaining available for future awards under the 2015 Plan, as of the effective date of the 2018 Plan. The number of shares of common stock reserved for issuance under the 2018 Plan will automatically increase on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 4% of the total number of shares of our stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2018 Plan is 45,000,000.

Prior to the Company’s IPO, the grant date fair value of the Company’s common stock was determined by the Company’s Board of Directors with the assistance of management and an independent third-party valuation specialist. The grant date fair value of the Company’s common stock was determined using valuation methodologies which utilizes certain assumptions including probability weighting of events, volatility, time to liquidation, a risk-free interest rate and an assumption for a discount for lack of marketability (Level 3 inputs). In determining the fair value of the Company’s common stock, the methodologies used to estimate the enterprise value of the Company were performed using methodologies, approaches, and assumptions consistent with the American Institute of Certified Public Accountants Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation (“AICPA Accounting and Valuation Guide”).

Subsequent to the Company’s IPO, the grant date fair value of each share of common stock underlying stock option awards is based on the closing price of our common stock as reported by the Nasdaq Select Global Market on the date of grant of the award.

The Company’s Board of Directors has the authority to determine to whom options will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.

2018 Employee Stock Purchase Plan

In September 2018, the Company’s board of directors approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”). The 2018 ESPP became effective on September 28, 2018. A total of 282,334 shares were initially reserved for issuance under the 2018 ESPP. Additionally, the number of shares of common stock reserved for issuance under the 2018 ESPP will increase automatically each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by the lesser of (1) 1% of the shares of common stock outstanding on December 31 of the preceding calendar year or (2) such lesser number of shares determined by the Company’s board of directors. The maximum number of shares that may be issued under the 2018 ESPP is 5,000,000. Our initial purchase period commences in the second half of 2019.

Valuation of Stock Options

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Three Months Ended March 31,
	2019			2018
Expected dividend yield	— %			—%
Expected term	6.03 years			6.07 years
Risk-free interest rate		2.55	%		2.65	%
Expected volatility		87	%		89	%

Using the Black-Scholes model, the weighted-average grant-date fair value of employee stock options granted was $8.82 and $0.34 per share during the three months ended March 31, 2019, and 2018, respectively.

Stock Option Activity

A summary of the 2015 Plan and 2018 Plan activity is as follows:

				Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2018		2,695,110			2,429,859		$	5.31		8.86	$	25,646
Authorized		1,160,000
Granted		(877,498	)		877,498		$	12.02
Exercised					(149,938	)	$	0.58
Cancelled		89,871			(89,871	)	$	8.19
Repurchased		3,381					$	0.35
Balance at March 31, 2019		3,070,864			3,067,548		$	7.38		9.01	$	19,527
Vested and exercisable – March 31, 2019					662,396		$	2.51		8.26	$	7,149
Vested and expected to vest – March 31, 2019					3,051,981		$	6.85		8.87	$	20,936

For the three months ended March 31, 2019 and 2018, the total intrinsic value of stock option awards exercised was $2.0 million and $0.02 million, respectively, determined at the date of option exercise, and the total cash received upon exercise of stock options was not significant for either period. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying stock option awards and the estimated fair value of the common stock on the date of exercise.

At March 31, 2019, $14.5 million of total unrecognized compensation cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 3.34 years. The total fair value of shares vested during the period ended March 31, 2019 was $1.0 million.

Stock-based compensation expense and awards granted to non-employees were not material for either the three months ended March 31, 2019 or March 31, 2018.

Stock-Based Compensation Expense

Total stock-based compensation for all awards granted to employees and consultants, before taxes is as follows (in thousands):

	Three Months Ended March 31,
	2019		2018
Research and development expenses	$	694	$	275
General and administrative expenses		313		79
Total	$	1,007	$	354

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Basis of Presentation

Reverse Stock Split

Need for Additional Capital

Use of Estimates

Other Risks and Uncertainties

Cash, Cash Equivalents, and Restricted Cash

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	34,572	$	52,183
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	35,564	$	53,175

Leases

The Company recognizes lease expense on a straight-line basis over the expected lease term.

Revenue Recognition

Recently Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows (in thousands):

	December 31, 2018			Adjustments due to Adoption of Topic 842			January 1, 2019
Property and equipment, net	$	29,494		$	(14,524	)	$	14,970
Operating lease right-of-use assets	$	-		$	14,224		$	14,224
Operating liabilities:
Lease liabilities, current portion	$	-		$	2,200		$	2,200
Accrued liabilities	$	992		$	(475	)	$	517
Deferred rent, net of current portion	$	1,353		$	(1,353	)	$	-
Lease financing obligation, net of current portion	$	10,490		$	(10,490	)	$	-
Lease liabilities, net of current portion	$	-		$	8,980		$	8,980
Accumulated deficit	$	(126,402	)	$	(153	)	$	(126,555	)

The impact of the adoption of Topic 842 on the accompanying condensed statements of operations was not material.

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2019

Accounting Policies [Abstract]

Cash, Cash Equivalents, and Restricted Cash

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the balance sheets that sum to the total of the same amounts shown in the statements of cash flows (in thousands).

	March 31,		December 31,
	2019		2018
Cash and cash equivalents	$	34,572	$	52,183
Restricted cash		992		992
Total cash, cash equivalents and restricted cash	$	35,564	$	53,175

Schedule of Impact of Topic 842 on Accompanying Condensed Balance Sheet and Statements of Operations

The impact of the adoption of Topic 842 on the accompanying condensed balance sheet as of January 1, 2019 was as follows (in thousands):

	December 31, 2018			Adjustments due to Adoption of Topic 842			January 1, 2019
Property and equipment, net	$	29,494		$	(14,524	)	$	14,970
Operating lease right-of-use assets	$	-		$	14,224		$	14,224
Operating liabilities:
Lease liabilities, current portion	$	-		$	2,200		$	2,200
Accrued liabilities	$	992		$	(475	)	$	517
Deferred rent, net of current portion	$	1,353		$	(1,353	)	$	-
Lease financing obligation, net of current portion	$	10,490		$	(10,490	)	$	-
Lease liabilities, net of current portion	$	-		$	8,980		$	8,980
Accumulated deficit	$	(126,402	)	$	(153	)	$	(126,555	)

Cash Equivalents and Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2019

Cash And Cash Equivalents [Abstract]

Cash Equivalents and Marketable Securities

The amortized cost, unrealized gains and losses and fair values of cash equivalents and marketable securities were as follows (in thousands):

	March 31, 2019
Description	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Money market funds	$	28,813	$	—	$	—	$	28,813
Commercial paper		55,212		48		—		55,260
Corporate debt securities		43,310		20		—		43,330
	$	127,335	$	68	$	—	$	127,403
Classified as:
Cash equivalents							$	29,813
Marketable securities								97,590
Total							$	127,403

	December 31, 2018
Description	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
Money market funds	$	36,148	$	—	$	—		$	36,148
Commercial paper		45,244		—		(40	)		45,204
Corporate debt securities		67,815		1		(46	)		67,770
	$	149,207	$	1	$	(86	)	$	149,122
Classified as:
Cash equivalents								$	48,195
Marketable securities									100,927
Total								$	149,122

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2019

Fair Value Disclosures [Abstract]

Assets and Liabilities Measured at Fair Value

The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands):

	March 31, 2019
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	28,813	$	28,813	$	—	$	—
Commercial paper		55,260		—		55,260		—
Corporate debt securities		43,330		—		43,330		—
Total	$	127,403	$	28,813	$	98,590	$	—

	December 31, 2018
Description	Total		Level 1		Level 2		Level 3
Money market funds	$	36,148	$	36,148	$	—	$	—
Commercial paper		45,204		—		45,204		—
Corporate debt securities		67,770		—		67,770		—
Total	$	149,122	$	36,148	$	112,974	$	—

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Components of Lease Costs

The components of lease costs, which were included in our condensed statements of operations and comprehensive loss, were as follows (in thousands):

	Three Months Ended March 31,
	2019
Lease cost
Operating lease cost	$	1,344
Short-term lease cost		56
Total lease cost	$	1,400

Schedule of Supplemental Information Related to Leases

Supplemental information related to leases was as follows (in thousands):

	Three Months Ended March 31,
	2019
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$	640
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$	10,542
Weighted average remaining lease term (years):
Operating leases		7.6
Weighted average discount rate:
Operating leases		9.0	%

Schedule of Minimum Annual Rental Payments Under Operating Lease Agreements

As of March 31, 2019, minimum annual rental payments under the Company’s operating lease agreements are as follows (in thousands):

	Operating Leases
Year ending December 31:
2019 (remaining nine months)	$	2,386
2020		5,058
2021		4,267
2022		3,623
2023		3,337
Thereafter		15,876
Total minimum payments	$	34,547
Less: Amounts representing interest		(10,836	)
Less: Amounts representing tenant improvement allowance		(4,033	)
Present value of future minimum lease payments		19,678
Less: Current portion of lease liability		(2,574	)
Noncurrent portion of lease liability	$	17,104

Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Fair Value of Stock Options Granted Estimated using Weighted Average Assumptions

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:

	Three Months Ended March 31,
	2019			2018
Expected dividend yield	— %			—%
Expected term	6.03 years			6.07 years
Risk-free interest rate		2.55	%		2.65	%
Expected volatility		87	%		89	%

Schedule of Stock Options Activity

A summary of the 2015 Plan and 2018 Plan activity is as follows:

				Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2018		2,695,110			2,429,859		$	5.31		8.86	$	25,646
Authorized		1,160,000
Granted		(877,498	)		877,498		$	12.02
Exercised					(149,938	)	$	0.58
Cancelled		89,871			(89,871	)	$	8.19
Repurchased		3,381					$	0.35
Balance at March 31, 2019		3,070,864			3,067,548		$	7.38		9.01	$	19,527
Vested and exercisable – March 31, 2019					662,396		$	2.51		8.26	$	7,149
Vested and expected to vest – March 31, 2019					3,051,981		$	6.85		8.87	$	20,936

Schedule of Stock Based Compensation Expense

Total stock-based compensation for all awards granted to employees and consultants, before taxes is as follows (in thousands):

	Three Months Ended March 31,
	2019		2018
Research and development expenses	$	694	$	275
General and administrative expenses		313		79
Total	$	1,007	$	354

Organization - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) Segment $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 $ / shares shares	Oct. 02, 2018 $ / shares shares
Organization And Nature Of Operations [Line Items]
Number of operating segments \| Segment		1
Net proceeds from issuance of stock \| $		$ 86	$ 9
Common stock, shares authorized		300,000,000		300,000,000
Common stock, par value \| $ / shares		$ 0.0001		$ 0.0001
Preferred stock, shares authorized		10,000,000		10,000,000
Preferred stock, par value \| $ / shares		$ 0.0001		$ 0.0001
IPO [Member]
Organization And Nature Of Operations [Line Items]
Number of common shares sold	6,854,202
Stock issued, price per share \| $ / shares	$ 15.00
Net proceeds from issuance of stock \| $	$ 92,600
Conversion of preferred stock shares into common stock shares	19,409,132
Reclassified common stock and additional paid in capital \| $	$ 177,900
Over-Allotment Option [Member]
Organization And Nature Of Operations [Line Items]
Number of common shares sold	187,535
Amended and Restated Certificate of Incorporation [Member]
Organization And Nature Of Operations [Line Items]
Common stock, shares authorized					300,000,000
Common stock, par value \| $ / shares					$ 0.0001
Preferred stock, shares authorized					10,000,000
Preferred stock, par value \| $ / shares					$ 0.0001

Summary of Significant Accounting Policies - Schedule of Impact of Topic 842 on Accompanying Condensed Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net	$ 15,652		$ 29,494
Operating lease right-of-use assets	22,018
Operating liabilities:
Lease liabilities, current portion	2,574
Accrued liabilities	803		992
Deferred rent, net of current portion			1,353
Lease financing obligation, net of current portion			10,490
Lease liabilities, net of current portion	17,104
Accumulated deficit	$ (144,564)		$ (126,402)
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net		$ 14,970
Operating lease right-of-use assets		14,224
Operating liabilities:
Lease liabilities, current portion		2,200
Accrued liabilities		517
Lease liabilities, net of current portion		8,980
Accumulated deficit		(126,555)
Accounting Standards Update 2016-02 [Member] \| Adjustments due to Adoption of Topic 842 [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, net		(14,524)
Operating lease right-of-use assets		14,224
Operating liabilities:
Lease liabilities, current portion		2,200
Accrued liabilities		(475)
Deferred rent, net of current portion		(1,353)
Lease financing obligation, net of current portion		(10,490)
Lease liabilities, net of current portion		8,980
Accumulated deficit		$ (153)

Commitments and Contingencies - Additional Information (Detail) $ in Thousands		1 Months Ended				3 Months Ended		12 Months Ended
	Jan. 01, 2019 USD ($)	Jan. 31, 2019 USD ($) renewal_term	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) ft²	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 2016 USD ($)	Nov. 30, 2015 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Operating lease cash security deposit						$ 1,021		$ 632
Operating lease right-of-use assets						22,018
Lease liability						19,678
Lessor financed improvement incentives due but not yet received									$ 1,200
Research and Development Expense						15,899	$ 11,401
Contract Research and Development Agreement [Member] \| Contract Research Organization [Member]
Commitments and Contingencies Disclosure [Line Items]
Upfront payment				$ 500
Term of agreement								1 year
Research and Development Expense							$ 300	$ 1,100
Maximum milestone payments				$ 36,400
Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating lease right-of-use assets	$ 14,224
Reduction to accumulated deficit						(153)
Emeryville California [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term		120 months									84 months
Operating lease cash security deposit		$ 600									$ 50
Operating leasing renewal option to extend lease		5 years									3 years
Operating lease for additional temporary space			12 months
Temporary space lease termination notice period			30 days
Operating leases number of renewal terms \| renewal_term		2
Tenant improvement allowance		$ 4,000
Operating lease right-of-use assets						10,400
Lease liability						$ 10,700
Lease expiration date						Oct. 31, 2019
Maximum lease termination effective period after rent commencement date under new lease						60 days
Adjustment to right-of-use assets and lease liability						$ 1,800
Cambridge Massachusetts [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing term			24 months							67 months
Operating lease cash security deposit			$ 300
Operating leasing renewal option to extend lease										3 years
Tenant improvement allowance										$ 2,100
Operating leases rent expense			100
Base rent paid for the year			$ 1,300
Cambridge Massachusetts [Member] \| Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating lease right-of-use assets	900
Cambridge Massachusetts [Member] \| Deposit [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating lease cash security deposit						300
Pleasanton [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating leasing renewal option to extend lease					5 years
Tenant improvement allowance						$ 1,200
Lease expiration date					Nov. 30, 2024
Net rentable area \| ft²					42,620
Letters of credit outstanding, amount					$ 1,000
Property plant and equipment additions								$ 9,300
Building improvements									6,100
Lessor financed improvement incentives due but not yet received									$ 1,200
Pleasanton [Member] \| Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies Disclosure [Line Items]
Operating lease right-of-use assets	4,000
Leased building derecognized	14,500
Financing obligation derecognized	10,500
Reduction to accumulated deficit	(200)
Pleasanton [Member] \| Prepaid Rent [Member] \| Accounting Standards Update 2016-02 [Member]
Commitments and Contingencies Disclosure [Line Items]
Unamortized tenant improvement allowance	$ 4,000

Collaboration and License Agreements - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		12 Months Ended
	Oct. 31, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued					0			0
Collaboration and license revenue					$ (1,347,000)
Deferred revenue					17,466,000			$ 18,813,000
Research and development					$ 15,899,000	$ 11,401,000
Series C [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common shares sold		768,115	153,360	690,128
Bluebird Bio Inc [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement early termination notice period					120 days
Early termination reduction of future milestone and royalty payments, description					Additionally, all of bluebird’s payment obligations that have not yet accrued related to future milestone and royalty payments will be reduced by fifty percent for the remainder of the agreement term.
Bluebird Bio Inc [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceeds from non refundable upfront payment		$ 20,000,000
Series C preferred stock equity investment received as consideration		$ 10,000,000
Offering price per share	$ 15.00
Additional preferred stock purchase	$ 10,000,000
Number of common shares sold	666,667
Development, regulatory, and commercial milestones					$ 1,200,000,000
Revenue recognition contract start month year					2018-08
Revenue recognition contract end month year					2023-08
Collaboration and license revenue					$ 1,300,000
Deferred revenue					17,500,000
Receivables					0
Contract assets					0
Bluebird Bio Inc [Member] \| Series C [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Preferred stock shares issued		768,115
Offering price per share		$ 13.04
Arbutus Biopharma Corporation and Protiva Biotherapeutics Inc [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment							$ 5,000,000
Reimbursed Expense							$ 200,000
Maximum milestone consideration payable					123,500,000
Development milestone payment					$ 2,500,000
Royalty consideration as a percentage of sales of licensed product					3.50%
Non Profit Hospital Cancer Center [Member] \| Collaboration Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development					$ 50,000,000
Common stock issued under warrant					40,257
Common stock issued under warrant exercise price					$ 0.35

Convertible Preferred Stock and Common Stock - Additional Information (Detail)	1 Months Ended				3 Months Ended
	Oct. 31, 2018 $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Jul. 31, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) Director $ / shares shares	Dec. 31, 2018 shares
Class of Stock [Line Items]
Preferred stock, shares authorized \| shares					10,000,000	10,000,000
Preferred stock, shares outstanding \| shares					0	0
Preferred stock redemption terms					A redemption event will only occur upon the liquidation or winding up of the Company, a greater than 50% change in control, or the sale of substantially all of the assets of the Company.
Cumulative dividends rate					8.00%
Dividend declared or accrued \| $					$ 0
Common stock, shares authorized \| shares					300,000,000	300,000,000
Common stock voting rights					Holders of common stock are generally entitled to one vote per share on all matters to be voted upon by the stockholders of the Company.
Dividends declared \| $					$ 0
Minimum [Member]
Class of Stock [Line Items]
Percentage of change in control, basis for redemption event					50.00%
IPO [Member]
Class of Stock [Line Items]
Stock issued, price per share	$ 15.00
Stock issued, number of shares \| shares	6,854,202
Conversion of preferred stock shares into common stock shares \| shares	19,409,132
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Stock issued, price per share		$ 13.04	$ 13.04	$ 13.04
Proceed from issuance of stock \| $		$ 10,000,000	$ 2,000,000	$ 8,900,000
Stock issued, number of shares \| shares		768,115	153,360	690,128
Payment for issuance cost \| $				$ 100,000
Preferred stock liquidation preference					$ 13.04
Preferred stock redemption price per share					$ 65.21
Number of directors to be elected by shareholders \| Director					1
Preferred stock conversion price					$ 13.04
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock liquidation preference					10.76
Preferred stock redemption price per share					$ 53.82
Number of directors to be elected by shareholders \| Director					1
Preferred stock conversion price					$ 10.76
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock liquidation preference					6.90
Preferred stock redemption price per share					$ 34.50
Number of directors to be elected by shareholders \| Director					3
Preferred stock conversion price					$ 6.90
Series A B And C Preferred Stock [Member] \| Minimum [Member]
Class of Stock [Line Items]
Preferred stock conversion price after amendment					15.66
Series A B And C Preferred Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Preferred stock conversion price before amendment					$ 21.53

Stock Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant-date fair value granted employee stock options granted	$ 8.82	$ 0.34
Intrinsic value of stock option awards exercised	$ 2,000	$ 20
Unrecognized compensation cost related to non-vested employee and consultant options	$ 14,500
Unrecognized compensation cost related to non-vested employee and consultant	3 years 4 months 2 days
Fair value of shares vested	$ 1,000
2018 Award Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for issuance			2,690,000
Increase in shares available for issuance	4.00%
Share-based Compensation Arrangement by Share-based Payment Award, maximum number of shares authorized	45,000,000
Minimum percentage of outstanding shares held by individual	10.00%
Minimum percentage fair market value of share price	110.00%
Option vesting period	4 years
Share based compensation by shared based payment award, description	If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.
2018 Award Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option granted expiration period	10 years
2018 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for issuance			282,334
Increase in shares available for issuance	1.00%
Share-based Compensation Arrangement by Share-based Payment Award, maximum number of shares authorized	5,000,000

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Options Available for Grant
Number of options beginning balance	2,695,110
Authorized	1,160,000
Granted	(877,498)
Cancelled	89,871
Repurchased	3,381
Number of options ending balance	3,070,864	2,695,110
Outstanding Options
Outstanding at beginning of period	2,429,859
Granted	877,498
Exercised	(149,938)
Cancelled	(89,871)
Outstanding at end of period	3,067,548	2,429,859
Options Outstanding, Number of Shares, Vested and exercisable	662,396
Options Outstanding, Number of Shares, Vested and expected to vest	3,051,981
Weighted Average Exercise Price
Beginning Balance	$ 5.31
Granted	12.02
Exercised	0.58
Cancelled	8.19
Repurchased	0.35
Ending Balance	7.38	$ 5.31
Weighted-Average Exercise Price, Vested and exercisable	2.51
Weighted-Average Exercise Price, Vested and expected to vest	$ 6.85
Options Outstanding, Weighted-Average Remaining Contractual Term (in years)	9 years 4 days	8 years 10 months 10 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and exercisable (in years)	8 years 3 months 4 days
Options Outstanding, Weighted-Average Remaining Contractual Term, Vested and expected to vest (in years)	8 years 10 months 14 days
Options Outstanding, Aggregate Intrinsic Value	$ 19,527	$ 25,646
Options Outstanding, Aggregate Intrinsic Value, Vested and exercisable	7,149
Options Outstanding, Aggregate Intrinsic Value, Vested and expected to vest	$ 20,936

Fair Value Measurements - Financial Assets and Liabilities Subject To Fair Value Measurements on a Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 127,403	$ 149,122
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	28,813	36,148
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	98,590	112,974
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	28,813	36,148
Money Market Funds [Member] \| Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	28,813	36,148
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	55,260	45,204
Commercial Paper [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	55,260	45,204
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	43,330	67,770
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets fair value	$ 43,330	$ 67,770

Property and Equipment, Net - Summary of Property and Equipment and Related Accumulated Depreciation and Amortization (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 22,592	$ 36,245
Less accumulated depreciation and amortization	(6,940)	(6,751)
Total property and equipment, net	15,652	29,494
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	619	470
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	974	935
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	17,548	16,406
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,081	3,063
Buildings and improvements capitalized under a lease financing transaction [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 15,371
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 370

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 1,036	$ 909

Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Prepaid research and development-related expenses	$ 1,461	$ 1,789
Prepaid insurance	650	966
Prepaid rent		860
Interest and other receivables	227	462
Other	500	449
Total prepaid expenses and other current assets	$ 2,838	$ 4,526

Balance Sheet Components - Schedule of Deposits and Other Long-Term Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Other Assets Unclassified [Abstract]
Lease security deposit	$ 1,021	$ 632
Restricted cash	992	992
Prepaid research and development-related expenses	554	554
Other	250	250
Total deposits and other long-term assets	$ 2,817	$ 2,428

Condensed Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	29,024,382	28,823,130
Common stock, shares outstanding	29,024,382	28,823,130

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands		3 Months Ended
	Sep. 20, 2018	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split	0.1449
Cash, cash equivalents and marketable securities		$ 132,200			$ 153,100
Accumulated deficit		144,564			$ 126,402
Net loss		18,009	$ 13,376
Net cash used in operating activities		$ 19,127	$ 15,060
Lessor financed improvement incentives due but not yet received						$ 1,200
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit				$ 126,555
Derecognized property, plant, and equipment				15,400
Derecognized property, plant, and equipment accumulated depreciation				900
Derecognized of financing lease obligation				$ 10,500

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost, Unrealized Gains and Losses and Fair Value of Cash Equivalent and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	$ 127,335	$ 149,207
Unrealized Gains	68	1
Unrealized Losses		(86)
Fair Value	127,403	149,122
Cash equivalents	29,813	48,195
Marketable securities	97,590	100,927
Money Market Funds [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	28,813	36,148
Fair Value	28,813	36,148
Commercial Paper [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	55,212	45,244
Unrealized Gains	48
Unrealized Losses		(40)
Fair Value	55,260	45,204
Corporate Debt Securities [Member]
Cash, cash equivalents and marketable securities [Line Items]
Amortized Cost	43,310	67,815
Unrealized Gains	20	1
Unrealized Losses		(46)
Fair Value	$ 43,330	$ 67,770

Fair Value Measurements - Additional Information (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, Assets, Level 1 to Level 2 Transfers	$ 0	$ 0
Fair Value, Liabilities, Level 1 to Level 2 Transfers	0	0
Fair Value, Assets, Level 2 to Level 1 Transfers	0	0
Fair Value, Liabilities, Level 2 to Level 1 Transfers	$ 0	$ 0

Commitments and Contingencies - Schedule of Components of Lease Costs (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Lease cost
Operating lease cost	$ 1,344
Short-term lease cost	56
Total lease cost	$ 1,400

Commitments and Contingencies - Schedule of Supplemental Information Related to Leases (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities (in thousands):
Operating cash flows from operating leases	$ 640
New right-of-use assets obtained in exchange for lease obligations (in thousands):
Operating leases	$ 10,542
Weighted average remaining lease term (years):
Operating leases	7 years 7 months 6 days
Weighted average discount rate:
Operating leases	9.00%

Commitments and Contingencies - Schedule of Minimum Annual Rental Payments Under Company's Operating Lease Agreements (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Operating Leases
2019 (remaining nine months)	$ 2,386
2020	5,058
2021	4,267
2022	3,623
2023	3,337
Thereafter	15,876
Total minimum payments	34,547
Less: Amounts representing interest	(10,836)
Less: Amounts representing tenant improvement allowance	(4,033)
Present value of future minimum lease payments	19,678
Less: Current portion of lease liability	(2,574)
Noncurrent portion of lease liability	$ 17,104

Collaboration and License Agreements - Schedule of Changes in Deferred Revenue Balance (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Revenue From Contract With Customer [Abstract]
Balance at January 1, 2019	$ 18,813
Additions	0
Deductions	(1,347)
Balance at March 31, 2019	$ 17,466

Net Loss Per Common Share - Computation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (18,009)	$ (13,376)
Denominator:
Weighted-average common shares outstanding, basic and diluted	28,938,891	2,217,726
Net loss per share, basic and diluted	$ (0.62)	$ (6.03)

Net Loss Per Common Share - Computation of Potentially Anti-dilutive Securities (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	3,239,820	19,998,208
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations		17,797,529
Options Issued and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	3,067,548	1,754,569
Early Exercised Common Stock Subject To Future Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities that were not included in the diluted per share calculations	172,272	446,110

Subsequent Event - Additional Information (Detail) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended
	May 09, 2019 USD ($) ft²	Apr. 30, 2019 USD ($) $ / shares shares	Oct. 31, 2018 shares	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Subsequent Event [Line Items]
Proceeds from issuance of common stock, net of issuance costs, net of underwriting discounts and commissions and offering costs \| $				$ 86	$ 9
Underwriters’ Option to Purchase Additional Shares [Member]
Subsequent Event [Line Items]
Stock issued, number of shares \| shares			187,535
Subsequent Event [Member]
Subsequent Event [Line Items]
Underwriters option exercise period		30 days
Proceeds from issuance of common stock, net of issuance costs, net of underwriting discounts and commissions and offering costs \| $		$ 69,600
Subsequent Event [Member] \| Pleasanton, California [Member]
Subsequent Event [Line Items]
Operating lease square feet of office space \| ft²	3,223
Operating leasing term	64 months
Annual base rent \| $	$ 100
Common Stock [Member] \| Subsequent Event [Member]
Subsequent Event [Line Items]
Shares issued price per share \| $ / shares		$ 11.50
Common Stock [Member] \| Subsequent Event [Member] \| Underwritten Public Offering [Member]
Subsequent Event [Line Items]
Stock issued, number of shares \| shares		6,500,000
Common Stock [Member] \| Subsequent Event [Member] \| Underwriters’ Option to Purchase Additional Shares [Member]
Subsequent Event [Line Items]
Maximum shares allowed to be purchased per underwriters 30 day option \| shares		975,000