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Summary of Significant Accounting Policies - Schedule of Adoption Of Topic 842 on Consolidated balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 01, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets:
Operating lease ROU assets	$ 52,775	$ 0
Liabilities:
Operating lease liabilities, current as included in other liabilities, current	1,866
Deferred rent, current as included in other liabilities, current				$ 712
Operating lease liabilities, noncurrent	$ 72,359
Deferred rent, noncurrent as included in other liabilities, noncurrent				11,041
Adjustment Due To Adoption Of Topic 842 [Member]
Assets:
Operating lease ROU assets				14,177
Liabilities:
Operating lease liabilities, current as included in other liabilities, current				1,176
Deferred rent, current as included in other liabilities, current				(712)
Operating lease liabilities, noncurrent				24,754
Deferred rent, noncurrent as included in other liabilities, noncurrent				$ (11,041)
Restatement Adjustment [Member]
Assets:
Operating lease ROU assets			$ 14,177
Liabilities:
Operating lease liabilities, current as included in other liabilities, current			1,176
Operating lease liabilities, noncurrent			$ 24,754

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 302,237	$ 275,357
Short-term marketable securities	307,732	202,177
Accounts receivable, net	2,637
Inventories	1,277
Prepaid expenses	9,422	6,337
Other assets, current	4,692	1,909
Total current assets	627,997	485,780
Property and equipment, net	27,113	14,981
Long-term marketable securities	85,030	114,281
Operating lease right-of-use assets	52,775
Restricted cash	2,395	2,395
Other assets, noncurrent	789	206
Total assets	796,099	617,643
Current liabilities:
Accounts payable	10,621	6,046
Accrued liabilities	41,358	16,792
Accrued compensation	17,578	10,036
Other liabilities, current	1,896	899
Total current liabilities	71,453	33,773
Long-term debt	73,559
Operating lease liabilities, noncurrent	72,359
Other liabilities, noncurrent	34	11,071
Total liabilities	217,405	44,844
Commitments and contingencies (Note 8)
Stockholders' equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized at December 31, 2019 and 2018, respectively, and none issued and outstanding as of December 31, 2019 and 2018
Common stock, $0.001 par value, 150,000,000 shares authorized at December 31, 2019 and 2018, respectively; 60,644,380 and 55,640,299 shares issued and outstanding at December 31, 2019 and 2018, respectively.	61	56
Additional paid-in capital	1,316,795	1,044,941
Accumulated other comprehensive income (loss)	754	(48)
Accumulated deficit	(738,916)	(472,150)
Total stockholders' equity	578,694	572,799
Total liabilities and stockholders' equity	$ 796,099	$ 617,643

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2016	$ 186,309	$ 37	$ 367,371	$ (166)	$ (180,933)
Beginning Balance, Shares at Dec. 31, 2016		36,638,156
Issuance of common stock upon equity offerings, net of issuance costs	231,955	$ 8	231,947
Issuance of common stock upon equity offerings, net of issuance costs, Shares		8,498,926
Issuance of common stock upon exercise of stock options	2,816	$ 1	2,815
Issuance of common stock upon exercise of stock options, Shares		578,455
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	(238)		(238)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		33,212
Issuance of common stock pursuant to ESPP purchases	1,050		1,050
Issuance of common stock pursuant to ESPP purchases, Shares		76,585
Vesting of restricted stock purchases	424		424
Vesting of restricted stock purchases, Shares		306,389
Stock-based compensation expense	13,682		13,682
Net unrealized gain (loss) on marketable securities	(170)			(170)
Net loss	(117,024)				(117,024)
Ending Balance at Dec. 31, 2017	318,804	$ 46	617,051	(336)	(297,957)
Ending Balance, Shares at Dec. 31, 2017		46,131,723
Issuance of common stock upon equity offerings, net of issuance costs	396,034	$ 8	396,026
Issuance of common stock upon equity offerings, net of issuance costs, Shares		8,403,826
Issuance of common stock upon exercise of stock options	6,022	$ 1	6,021
Issuance of common stock upon exercise of stock options, Shares		596,434
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	(6,252)	$ 1	(6,253)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		255,039
Issuance of common stock pursuant to ESPP purchases	1,647		1,647
Issuance of common stock pursuant to ESPP purchases, Shares		61,031
Vesting of restricted stock purchases	369		369
Vesting of restricted stock purchases, Shares		192,246
Stock-based compensation expense	30,080		30,080
Net unrealized gain (loss) on marketable securities	288			288
Net loss	(174,193)				(174,193)
Ending Balance at Dec. 31, 2018	572,799	$ 56	1,044,941	(48)	(472,150)
Ending Balance, Shares at Dec. 31, 2018		55,640,299
Issuance of common stock upon equity offerings, net of issuance costs	219,671	$ 4	219,667
Issuance of common stock upon equity offerings, net of issuance costs, Shares		3,986,890
Issuance of common stock upon exercise of stock options	$ 11,636	$ 1	11,635
Issuance of common stock upon exercise of stock options, Shares	538,502	538,503
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	$ (7,617)		(7,617)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		368,357
Issuance of common stock pursuant to ESPP purchases	2,361		2,361
Issuance of common stock pursuant to ESPP purchases, Shares		63,280
Vesting of restricted stock purchases	157		157
Vesting of restricted stock purchases, Shares		47,051
Stock-based compensation expense	45,651		45,651
Net unrealized gain (loss) on marketable securities	802			802
Net loss	(266,766)				(266,766)
Ending Balance at Dec. 31, 2019	$ 578,694	$ 61	$ 1,316,795	$ 754	$ (738,916)
Ending Balance, Shares at Dec. 31, 2019		60,644,380

Stockholders' Equity - Additional Information (Detail)	12 Months Ended
Stockholders' Equity - Additional Information (Detail)	Dec. 31, 2019 USD ($)
Equity [Abstract]
Common stock dividend declared	$ 0

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America, or U.S. GAAP.

Use of Estimates

The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities, the

disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions.

Principles of Consolidation

The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiary. All significant intercompany transactions and balances have been eliminated upon consolidation.

Segment Reporting

We have determined that we operate in a single segment based upon the way the business is organized for making operating decisions and assessing performance. We have only one operating segment related to the development of pharmaceutical products. All property and equipment is maintained in the United States.

Cash and Cash Equivalents

We consider all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value.

Investments in Marketable Securities

We invest in marketable securities, primarily money market funds, corporate debt securities, government securities, government agency securities, and certificates of deposits. We classify our marketable securities as

available-for-sale

securities and report them at fair value in cash equivalents or marketable securities on the consolidated balance sheets with related unrealized gains and losses included within accumulated other comprehensive income (loss) on the consolidated balance sheet. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the consolidated statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on

available-for-sale

securities are included in interest and other income (loss). The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as

available-for-sale

are included in interest income.

We regularly review all of our investments for other-than-temporary declines in estimated fair value. Our review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. When we determine that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, we reduce the carrying value of the security and record a loss for the amount of such decline.

Fair Value Measurement

The carrying amounts of certain financial instruments, including cash and cash equivalents, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities.

Concentration of Risk

Credit Risk

We invest in a variety of financial instruments and, by our Board approved investment policy, limit the amount of credit exposure with any one issuer, industry or geographic area for investments other than instruments backed by the U.S. federal government.

Major Customers

We have entered into distribution agreements with certain limited specialty pharmacies and a specialty distributor. For the year ended December 31, 2019, our two largest customers represented approximately 90% of our product revenue.

Major Suppliers

We do not currently have any of our own manufacturing facilities, and therefore depend on an outsourced manufacturing strategy for the production of Oxbryta for commercial use and for the production of our product candidates for clinical trials. We have contracts in place with one third-party manufacturer that is approved for the commercial production of Oxbryta and one third-party supplier that is approved for Oxbryta’s active pharmaceutical ingredient. Although there are potential sources of supply other than our existing manufacturers and suppliers, any new supplier would be required to qualify under applicable regulatory requirements.

Accounts Receivables, net

Accounts receivables are recorded net of estimates of variable consideration for which reserves are established and which result from discounts and chargebacks that are offered within contracts between us and a limited number of specialty pharmacies and a specialty distributor in the United States. These reserves are classified as reductions of accounts receivable.

We estimate the allowance for doubtful accounts based on an evaluation of aging of our receivables. Accounts receivable balances are written off against the allowance when it is probable that the receivable will not be collected. Given the nature and history of our accounts receivable, we determined that an allowance for doubtful accounts was not required at December 31, 2019.

Inventories

Inventories are stated at the lower of cost or estimated net realizable value, on a

first-in,

first out, or FIFO, basis. We use actual costs to determine our cost basis for inventories. Inventories consist of raw materials,

work-in-process,

and finished goods.

We began capitalizing costs as inventory when the product candidate received regulatory approval. Prior to regulatory approval, we recorded inventory costs related to product candidates as research and development expenses.

We periodically assess the recoverability of our inventory and reduce the carrying value of the inventory when items are determined to be obsolete, defective or in excess of forecasted sales requirements. Inventory write-downs for excess, defective and obsolete inventory are recorded as a cost of sales. There have been no write-downs of our inventories for the periods presented.

Property and Equipment, Net

Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the assets, which is three years for

computer equipment and five years for laboratory equipment. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged as expense in the statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations.

Impairment of Long-Lived Assets

We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. There have been no impairments of our long-lived assets for the periods presented.

Restricted Cash

Restricted cash consists of cash deposits held by our financial institution as collateral for our letter of credit under our facility lease.

Accruals of Research and Development and Manufacturing Costs

We record accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development. These costs are a significant component of our research and development expenses. A substantial portion of our ongoing research and development activities are conducted by third-party service providers, including contract research organizations and contract manufacturing organizations. We also accrue for estimated costs of manufacturing activities for inventories. These costs are a significant component of the cost of our inventory.

We accrue the costs incurred under our agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. We determine the accruals for research and development costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the clinical studies and the agreed-upon fee to be paid for such services.

The accrual for contract manufacturing activities is based on an estimate of manufacturing activities completed to date, contractual rates, and amounts invoiced and paid to date at the end of each reporting period. We determine the percentage of manufacturing activities completed to date based on discussions with the contract manufacturing organization, oversight of the manufacturing activities and anticipated timeline.

As actual costs become known, we adjust our accruals. We have not experienced any material deviations between accrued costs and actual costs. However, actual clinical and manufacturing services performed, number of subjects enrolled, and the rate of subject enrollment may vary from our estimates, resulting in adjustments to research and development costs or inventories in future periods. Changes in these estimates that result in material changes to our accruals of research and development and manufacturing costs could materially affect our results of operations or amounts of inventories capitalized.

Revenue Recognition

Pursuant to Accounting Standards Codification, Topic 606,

Revenue from Contracts with Customers

or ASC 606

we recognize revenue upon transfer of control of promised products or services to customers in an amount that reflects the consideration we expect to receive in exchange for those products or services.

To determine revenue recognition for arrangements that we determine are within the scope of ASC 606, we perform the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) we satisfy a performance obligation. We only apply the five-step model to contracts when it is probable that we will collect substantially all of the consideration we are entitled to in exchange for the goods or services we transfer to the customer.

Product sales, net

Our product sales consist of U.S. sales of Oxbryta, which we began shipping to customers in December 2019. Prior to December 2019, we had no product sales. We sell Oxbryta to a limited number of specialty pharmacies and a specialty distributor, or collectively, Customers. These agreements with our Customers provide for transfer of title to the product at the time the product has been delivered to the Customers. The Customers subsequently dispense our products directly to a patient or resell our products to hospitals and certain pharmacies.

We recognize revenue on product sales when the Customers obtain control of our product, which occurs at a point in time, typically upon delivery to our Customers. It is at that point that we have a right to payment and that our Customers obtain title and the risks and rewards of ownership. Shipping and handling activities are considered to be fulfillment activities rather than a separate performance obligation. Payment terms are typically

30-60

days following delivery to our Customers. As allowed under ASC 606 via practical expedient, because payment is expected shortly after delivery, we do not adjust the amount of consideration expected to be received for the effects of a significant financing component.

We consider the effects of items that can decrease the transaction price, such as variable consideration and consideration payable to Customers or payer. Amounts related to such items are estimated at contract inception and updated at the end of each reporting period as additional information becomes available. The amount of variable consideration may be constrained and is included in the transaction price only to the extent it is probable that a significant reversal of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is resolved. Revenue from product sales is recorded after considering the impact of the following variable consideration amounts along with the constraint at the time of revenue recognition:

Rebates:

We are subject to government mandated rebates for Medicaid Drug Rebate Program, Medicare Part D Prescription Drug Benefit Program, and other government health care programs in the United States. Rebate amounts are based upon contractual agreements or legal requirements with public sector benefit providers. We use the expected-value method for estimating these rebates based on statutory discount rates and expected utilization. The expected utilization of rebates is estimated based on third party market research data and data received from the specialty pharmacies and specialty distributor. Estimates for these rebates are adjusted quarterly to reflect the most recent information. We record an accrued liability for unpaid rebates related to products for which control has been transferred to Customers.

Prompt payment discounts

: We provide discounts to our Customers if they pay for our products within a defined period of time after title transfers, which terms are explicitly stated in the contract. We use the most-likely-amount method for estimating prompt payment discounts. We expect that our Customers will earn prompt payment discounts. As a result, we deduct the full amount of those discounts from total product sales when revenues are recognized and record these discounts as a reduction of accounts receivable.

Co-payment assistance:

We provide

co-payment

assistance to patients who have commercial insurance and meet certain eligibility requirements. We use the expected-value method for estimating

co-payment

assistance based on estimates of program redemption using data provided by third-party administrators. Estimates for the

co-payment

assistance are adjusted quarterly to reflect actual experience. We record an accrued liability for unredeemed

co-payment

assistance related to products for which control has been transferred to Customers.

Medicare Part D Coverage Gap:

The Medicare Part D coverage gap is a federal program to subsidize the costs of prescription drugs for Medicare beneficiaries in the United States, which mandates manufacturers to fund a portion of the Medicare Part D insurance coverage gap for prescription drugs sold to eligible patients. Funding of the coverage gap is generally invoiced and paid in arrears. We estimate the impact of the Medicare Part D coverage gap using the expected-value method based on an amount expected to be incurred for the current quarter’s activity, plus an accrual balance for known prior quarters. Estimates for the impact of the Medicare Part D coverage gap are adjusted quarterly to reflect actual experience. We record an accrued liability for unpaid reserves related to the Medicare Part D coverage gap.

Product returns:

Consistent with industry practice, we offer limited product return rights and generally allow for the return of product that is damaged or defective, or within a few months prior to and up to a few months after the product expiration date. We consider several factors in the estimation of potential product returns, including expiration dates of the product shipped, the limited product return rights, third-party data in monitoring channel inventory levels, shelf life of the product, prescription trends, and other relevant factors. We expect product returns to be immaterial. Other than these limited returns, we do not provide any product warranties.

Chargebacks

: Chargebacks are discounts that occur when contracted parties purchase directly from a specialty distributor. Contracted parties, which currently consist primarily of Public Health Service Institutions and federal government entities purchasing via the Federal Supply Schedule, generally purchase the product at a discounted price. The specialty distributor, in turn, charge back the difference between the price initially paid by the specialty distributor and the discounted price paid to the specialty distributor by the contracted parties to us. The reserves for chargeback are based on known sales to contracted parties. We establish the reserves for chargebacks in the same period that the related revenue is recognized, resulting in a reduction of product revenue and receivables.

Distributor fees:

Our specialty distributor provides distribution services to us for a fee, based on a contractually determined fixed percentage of sales. We estimate these distributor fees and record such estimates in the same period the related revenue is recognized, resulting in a reduction of product revenue. We record an accrued liability for unpaid distributor fees.

Other revenue recognition considerations

Oxbryta is our only product. The only performance obligation included in our contracts is the delivery of Oxbryta to our Customers. Therefore, no allocation of transaction price amongst performance obligations is necessary. Consequently, the transaction price determined after considering the impacts of variable consideration is recognized at the time control is transferred to our Customers, which is upon delivery of Oxbryta to our Customers.

Because all sales of Oxbryta are in the United States and because our Customers are each a large distributor with similar variable consideration impacts, we provide revenue numbers on a total basis without further disaggregation. Additionally, we do not have any contract assets or liabilities, other than accounts receivable, related to our sales of Oxbryta.

Cost of Sales

Cost of sales consists primarily of direct and indirect costs related to the manufacturing of Oxbryta products sold, including third-party manufacturing costs, packaging services, freight, storage costs, allocation of overhead costs of employees involved with production, and Oxbryta net sales-based royalties payable to the Regents of the University of California. Costs incurred prior to FDA approval of Oxbryta in November 2019 have been recorded as research and development expense in our consolidated statement of operations.

Leases

Leases (Topic 842) Effective January 1, 2019

We determine if an arrangement is or contains a lease at inception by assessing whether the arrangement contains an identified asset and whether we have the right to control the identified asset.

Right-of-use,

or ROU, assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. Lease liabilities are recognized at the lease commencement date based on the present value of future lease payments over the lease term. ROU assets are based on the measurement of the lease liability and also include any lease payments made prior to or on lease commencement and exclude lease incentives received and initial direct costs incurred, as applicable.

As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at the commencement date in determining the present value of future payments. We consider our credit risk, term of the lease, and total lease payments and adjust for the impact of collateral, as necessary, when calculating our incremental borrowing rates. The lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise any such options. Lease cost for our operating leases is recognized on a straight-line basis over the lease term.

We have elected to not separate lease and

non-lease

components for any leases within its existing classes of assets and, as a result, account for any lease and

non-lease

components as a single lease component. We have also elected to not recognize any leases within its existing classes of assets with a term of 12 months or less.

ROU assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and estimated incremental borrowing rate upon lease modification.

Leases (Topic 840) Prior to the Adoption of Topic 842

We enter into lease agreements for our office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the noncancelable term of the lease and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability, which is included within other liabilities on the consolidated balance sheet. Incentives granted under our facilities leases, including rent holiday and allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the noncancelable term of the lease.

Comprehensive Income (Loss)

Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from

non-owner

sources. Our comprehensive income (loss) is comprised of net loss and changes in unrealized gains and losses on our marketable securities.

Research and Development

Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on our behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed.

Long-term Debt

Long-term debt consists of our loan agreement, or Term Loan, with funds managed by Pharmakon Advisors LP, which are BioPharma Credit PLC, as collateral agent and a lender, and Biopharma Credit Investments V

(Master) LP, as a lender, and collectively, the Lenders. We accounted for the Term Loan as a debt financing arrangement. Interest expense is accrued using the effective interest rate method over the estimated period of the debt will be repaid. Debt issuance costs have been recorded as a debt discount in our consolidated balance sheets and are being amortized and recorded as interest expense throughout the life of the Term Loan using the effective interest rate method. We consider whether there are any embedded features in our debt instruments that require bifurcation and separate accounting as derivative financial instruments pursuant to Accounting Standards Codification, or ASC, Topic 815,

Derivatives and Hedging

Stock-Based Compensation

We measure and recognize stock-based compensation expense, including employee and

non-employee

equity awards, based on fair value at the grant date. We use the Black-Scholes-Merton option-pricing model to calculate fair value. Stock-based compensation expense recognized in the consolidated statements of operations is based on stock awards ultimately vested, taking into consideration actual forfeitures.

Income Taxes

We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance.

We recognize benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. To date, there have been no interest or penalties incurred in relation to the unrecognized tax benefits.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given our net loss.

Recent Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No.

2018-15,

Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing

Arrangement That Is a Service Contract

, or ASU

2018-15.

ASU No.

2018-15

requires a customer that is a party to a cloud computing service contract to follow the

internal-use

software guidance in Subtopic

350-40

to determine which implementation costs to capitalize and which costs to expense. The amendments in this update are effective for annual reporting periods beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption of the amendments in this update is permitted. The amendments in this update should be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. We believe that the adoption of this new standard will have no material impact on our consolidated financial position or results of operations and have not elected to early adopt the amendment.

In August 2018, the FASB issued ASU No.

2018-13,

Fair Value Measurement (Topic 820)

. The new standard modifies the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement, including removals of, modification to, and additional disclosure requirements from Topic 820. The amendment of ASU No.

2018-13

removes disclosure requirements from Topic 820 in the areas of (1) the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, (2) the policy for timing of transfers between levels, and (3) the valuation processes for Level 3 fair value measurements. The amendments in this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Except for certain amendments related to Level 3 fair value measurements, all the other amendments should be applied retrospectively to all periods presented upon their effective date. Early adoption is permitted upon issuance of ASU No.

2018-13.

We believe that the adoption of this new standard will have no material impact on our consolidated financial position or results of operations and have not elected to early adopt the amendment.

In June 2016, the FASB issued ASU No.

2016-13,

Measurement of Credit Losses on Financial Instruments (Topic 326)

, which amends the guidance on the impairment of financial instruments. The new standard adds to U.S. GAAP an impairment model that is based on expected losses rather than incurred losses, which is known as the current expected credit loss, or CECL model. The CECL model applies to most debt instruments (other than those measured at fair value), trade and other receivables, financial guarantee contracts, and loan commitments. The amendments in this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption of the amendments in this update is permitted. We believe that the adoption of this new standard will have no material impact on our consolidated financial position or results of operations and have not elected to early adopt the amendment.

Accounting Pronouncements Adopted

Leases (Topic 842)

In February 2016 the FASB issued ASU

2016-02,

Leases (Topic 842 or ASU

2016-02).

ASU

2016-02

amends several aspects of lease accounting, including requiring lessees to recognize almost all leases with a term greater than one year as a ROU asset and corresponding liability, measured at the present value of the lease payments. On January 1, 2019, we adopted Topic 842 using the modified retrospective approach as of the adoption date. Results for the three and nine months ended September 30, 2019 are presented under Topic 842. No prior period amounts were adjusted and continue to be reported in accordance with previous lease guidance, Accounting Standards Codification Topic 840, Leases, or Topic 840.

The new standard provides a number of optional practical expedients in transition. We elected the practical expedients to not reassess our prior conclusions about lease identification under the new standard, to not reassess lease classification, and to not reassess initial direct costs. We did not elect the practical expedient allowing the use of hindsight, which would require us to reassess the lease term of our 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 our current contract portfolio.

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


	December 31, 2018		Adjustments Due to our Adoption of Topic 842			January 1, 2019
Assets:
Operating lease ROU assets	$	—	$	14,177		$	14,177
Liabilities:
Operating lease liabilities, current as included in other liabilities, current		—		1,176			1,176
Deferred rent, current as included in other liabilities, current		712		(712	)		—
Operating lease liabilities, noncurrent		—		24,754			24,754
Deferred rent, noncurrent as included in other liabilities, noncurrent		11,041		(11,041	)		—

The adjustments due to our adoption of Topic 842 related to the recognition of ROU assets and lease liabilities for the existing operating leases. A cumulative-effect adjustment to beginning retained earnings was not required.

Other accounting pronouncements adopted

In February 2018, the FASB issued ASU No.

2018-02,

Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income

. The amendment of ASU No.

2018-02

states an entity may elect to reclassify the income tax effects of the Tax Cuts and Jobs Act of 2017, or Tax Cuts and Jobs Act, on items within accumulated other comprehensive income to retained earnings. The amendments in this update are effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. Early adoption is permitted. We adopted ASU No.

2018-02

in the first quarter of 2019. The adoption of this new standard did not have a material impact on our consolidated financial position or results of operations.

Income Taxes - Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	21.00%	34.00%
State taxes	9.70%	1.30%
Federal and state tax credits	4.90%	7.10%	7.30%
Change in valuation allowance	(37.20%)	(33.30%)	(14.80%)
Foreign rate differential		1.70%	(4.60%)
Officer compensation limitation	(1.00%)	(0.70%)	(0.90%)
Stock based compensation/Non-deductible changes in fair value	1.70%	2.90%	0.70%
Liquidation of foreign entities	0.90%
Tax reform—tax rate change			(21.70%)
Provision for Taxes	0.00%	0.00%	0.00%

Commitments and Contingencies - Supplemental cash flow information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Oct. 01, 2019
Commitment And Contingencies [Line Items]
Adjustment to ROU assets as a result of the lease modification for the Existing Premises	$ (13,802)
ROU assets obtained for new operating lease liabilities	53,727
Cash paid for amounts included in the measurement of lease liabilities	$ 4,481
Weighted-average remaining lease term of operating leases (in years)	10 months 3 days
Weighted-average discount rate of operating leases	8.66%	8.78%
Accounting Standards Update 2016-02 [Member]
Commitment And Contingencies [Line Items]
ROU assets obtained in exchange for new operating lease upon adoption of ASC 842	$ 14,177

Stockholders' Equity

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Stockholders' Equity

9. Stockholders’ Equity

Common Stock Reserved for Issuance

We have reserved sufficient shares of common stock for issuance upon the exercise of stock options, vesting of restricted stock units and restricted shares subject to future vesting. Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of any preferred stockholders. As of December 31, 2019, no common stock dividends had been declared by the board of directors.

We have reserved shares of common stock for future issuance as follows:


	December 31,
	2019		2018
Restricted shares subject to future vesting		—		47,051
Restricted stock units		1,848,772		975,419
Options issued and outstanding		3,573,860		3,243,551
Shares available for future grant under the 2015 Plan and 2017 Inducement Equity Plan		4,478,656		3,003,454
Employee stock purchase plan		252,655		240,935

Total		10,153,943		7,510,410

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
U.S. corporate income tax rate	21.00%	21.00%	34.00%
Provision for income taxes	$ 0	$ 0	$ 0
Increase in valuation allowance	99,200,000	57,900,000	$ 17,800,000
Deferred tax assets, valuation allowance	$ 245,000,000	$ 145,788,000
valuation allowance charged on deferred tax assets	100.00%
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
U.S. corporate income tax rate		21.00%	35.00%
State [Member]
Income Taxes [Line Items]
Operating loss carry forward, expiration year	2039
State [Member] \| Operating Loss Carryforward Expiration Year 2036 [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 386,800,000
Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 618,200,000
Operating loss carry forward, expiration year	2037
Domestic Tax Authority [Member] \| Tax period 2037 [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 209,900,000
Domestic Tax Authority [Member] \| Operating Loss Carryforward With Indefinite Expiry Period [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 408,300,000

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance on Converted basis (Detail) - shares	Dec. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Shares reserved for future issuance	10,153,943	7,510,410
Restricted Stock [Member]
Class of Stock [Line Items]
Shares reserved for future issuance		47,051
RSU [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	1,848,772	975,419
Stock Options [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	3,573,860	3,243,551
Shares Available for Future Grant Under the 2015 Plan and 2017 Inducement Equity Plan [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	4,478,656	3,003,454
ESPP [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	252,655	240,935

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 16,232	$ 11,150
Additions based on tax positions related to current year	5,366	5,144
Decreased for prior period positions		(62)
Unrecognized tax benefit - December 31	$ 21,598	$ 16,232

Available-for-Sale Securities (Tables)

12 Months Ended

Dec. 31, 2019

Investments, Debt and Equity Securities [Abstract]

Summary of Available-for-Sale Securities

The following table is a summary of

available-for-sale

securities recorded in cash and cash equivalents, or marketable securities in our Consolidated Balance Sheets (in thousands):


	December 31, 2019									December 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized (Losses)			Estimated Fair Value		Amortized Cost		Unrealized Gains		Unrealized (Losses)			Estimated Fair Value
Financial Assets:
Money market funds	$	250,535	$	—	$	—		$	250,535	$	275,234	$	—	$	—		$	275,234
Corporate debt securities		151,773		384		(8	)		152,149		110,053		69		(95	)		110,027
U.S. government agency securities		94,963		73		(4	)		95,032		88,042		40		(54	)		88,028
Certificates of deposits		6,239		43		—			6,282		6,681		1		(7	)		6,675
U.S. government securities		139,978		266		—			140,244		111,730		60		(62	)		111,728

Total	$	643,488	$	766	$	(12	)	$	644,242	$	591,740	$	170	$	(218	)	$	591,692

Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets

The following table summarizes the classification of the

available-for-sale

securities on our Consolidated Balance Sheets (in thousands):


	December 31, 2019		December 31, 2018
Cash and cash equivalents	$	251,480	$	275,234
Short-term marketable securities		307,732		202,177
Long-term marketable securities		85,030		114,281

Total	$	644,242	$	591,692

Inventories - Summary of Capitalizing Inventories (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
Inventory, Raw Materials and Supplies, Gross [Abstract]
Raw materials	$ 700
Work-in-process	525
Finished goods	52
Total inventories	$ 1,277

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss before provision for income taxes:
United States	$ (266,595)	$ (174,190)	$ (101,288)
International	(167)		(15,736)
Loss before provision for income taxes	$ (266,762)	$ (174,190)	$ (117,024)

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring Basis

The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands):


	December 31, 2019
	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	250,535	$	250,535	$	—	$	—
Corporate debt securities		152,149		—		152,149		—
U.S. government agency securities		95,032		—		95,032		—
Certificates of deposits		6,282		—		6,282		—
U.S. government securities		140,244		—		140,244		—

Total financial assets	$	644,242	$	250,535	$	393,707	$	—


	December 31, 2018

	Total		Level 1		Level 2		Level 3
Financial Assets:
Money market funds	$	275,234	$	275,234	$	—	$	—
Corporate debt securities		110,027		—		110,027		—
U.S. government agency securities		88,028		—		88,028		—
Certificates of deposits		6,675		—		6,675		—
U.S. government securities		111,728		—		111,728		—

Total financial assets	$	591,692	$	275,234	$	316,458	$	—

Commitments and Contingencies - Lessee Operating Lease Liability Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 01, 2019
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2020	$ (2,440)
2021	11,736
2022	12,116
2023	12,508
2024	12,913
2025	13,333
Thereafter	61,035
Total lease payments	121,201
Less: Imputed interest	(46,976)
Present value of operating lease liabilities	$ 74,225	$ 2,900

Share-based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, outstanding, Number of Options	3,243,551
Options granted, Number of Options	1,026,965
Options exercised, Number of Options	(538,502)
Options canceled, Number of Options	(158,154)
Ending balance, outstanding, Number of Options	3,573,860	3,243,551
Vested and exercisable, Number of Options	1,884,402
Weighted- Average Exercise Price
Beginning balance, Outstanding, Weighted-Average Exercise Price	$ 29,740
Options granted, Weighted-Average Exercise Price	50,040.00
Options exercised, Weighted-Average Exercise Price	21,610
Options canceled, Weighted-Average Exercise Price	42,260
Ending balance, Outstanding, Weighted-Average Exercise Price	36,240	$ 29,740
Vested and exercisable, Weighted-Average Exercise Price	$ 27,950
Beginning, Outstanding, Weighted-Average remaining contractual term	7 years 6 months 18 days	7 years 11 months 15 days
Vested and exercisable, Weighted-Average remaining contractual term	6 years 8 months 15 days
Balance Outstanding, Aggregate Intrinsic Value	$ 154,552
Vested and exercisable, Aggregate Intrinsic Value	$ 97,114

Share-based Compensation - Valuation Assumptions for Stock Options Granted (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	69.80%	68.70%	68.90%
Volatility, maximum	72.20%	71.80%	75.60%
Risk-free interest rate, minimum	1.40%	2.60%	1.80%
Risk-free interest rate, maximum	2.60%	3.00%	2.30%
Stock Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 3 months 18 days	5 years 3 months 18 days
Stock Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Long-term Debt-Schedule of Future payments of principal and interest on the Term Loan (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Contractual Obligation, Fiscal Year Maturity Schedule [Abstract]
2020	$ 6,750
2021	6,750
2022	6,750
2023	31,406
2024	29,156
2025	26,906
Total minimum payments	107,718
Less amount representing interest	(31,218)
Less amount representing Paydown Fee	(1,500)
Long-term debt, gross	75,000
Discount on notes payable	(1,468)
Accretion of Paydown Fee	27
Long-term debt	$ 73,559

Share-based Compensation - Stock-based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 45,651	$ 30,080	$ 13,682
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	19,140	12,747	5,905
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 26,511	$ 17,333	$ 7,777

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

12. Income Taxes

December 2017, the President signed the Tax Cuts and Jobs Act

Tax Act. The Tax Act, among other things, lowered the U.S. corporate income tax rate from 35% to 21% effective January 1, 2018. Consequently, our gross deferred tax assets as of December 31, 2017 were significantly reduced to reflect the estimated impact of the Tax Act. Due to our lack of earnings history and uncertainties surrounding our ability to generate future taxable income, the net deferred tax assets have been fully offset by a valuation allowance. The significant reduction in our gross deferred tax assets are fully offset by a reduction in valuation allowance, resulting in no impact to our income tax expense.

The components of the loss before income taxes were as follows (in thousands):


	Year Ended December 31,
	2019			2018			2017
Loss before provision for income taxes:
United States	$	(266,595	)	$	(174,190	)	$	(101,288	)
International		(167	)		—			(15,736	)

	$	(266,762	)	$	(174,190	)	$	(117,024	)

No provision for income taxes was recorded for the years ended December 31, 2019, December 31, 2018 and December 31, 2017. We have incurred net operating losses for all the periods presented. We have not reflected any benefit of such net operating loss (NOL) carryforwards in the accompanying consolidated financial statements. We have established a full valuation allowance against the related deferred tax assets due to the uncertainty surrounding the realization of such assets.

The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows:


	Year Ended December 31,
	2019			2018			2017
Federal statutory income tax rate		21.0	%		21.0	%		34.0	%
State taxes		9.7			1.3			—
Federal and state tax credits		4.9			7.1			7.3
Change in valuation allowance		(37.2	)		(33.3	)		(14.8	)
Foreign rate differential		—			1.7			(4.6	)
Officer compensation limitation		(1.0	)		(0.7	)		(0.9	)
Stock based compensation/Non-deductible changes in fair value		1.7			2.9			0.7
Liquidation of foreign entities		0.9			—			—
Tax reform—tax rate chang e		—			—			(21.7	)

Provision for Taxes		0.0	%		0.0	%		0.0	%

The components of the deferred tax assets and liabilities are as follows (in thousands):


	December 31,
	2019			2018
Deferred tax assets:
Net operating loss carryforwards	$	157,685		$	90,861
Tax credits		62,862			47,374
Operating lease liability		16,177			2,471
Accruals and reserves		3,996			1,954
Stock based compensation		11,166			4,838
Intangibles		8,243			—
Other		408			—

Gross deferred tax assets		260,537			147,498
Valuation allowance		(245,000	)		(145,788	)

Net deferred tax assets		15,537			1,710
Operating lease – ROU asset		(14,971	)		—
Property and equipment		(352	)		(1,710	)
Other		(214	)		—
Gross deferred tax liabilities		(15,537	)		(1,710	)
Net deferred tax assets	$	—		$	—

Realization of the deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. We have established a valuation allowance to offset deferred tax assets as of December 31, 2019 and 2018 due to the uncertainty of realizing future tax benefits from our net operating loss carryforwards and other deferred tax assets. The valuation allowance increased approximately $99.2 million, $57.9 million, and $17.8 million during the years ended December 31, 2019, 2018, and 2017, respectively. The increase in the valuation allowance is mainly related to the increase in net operating loss carryforwards and the increase in tax credits generated during the respective taxable years.

As of December 31, 2019, we had federal net operating loss carryforwards of approximately $618.2 million to offset future federal taxable income, with $209.9 million available through 2037 and $408.3 million available indefinitely. We also had state net operating loss carryforwards of approximately $386.8 million that may offset future state taxable income through 2039.

Current federal and state tax laws include substantial restrictions on the utilization of net operating losses and tax credits in the event of an ownership change. Even if the carryforwards are available, they may be subject to annual limitations, lack of future taxable income, or future ownership changes that could result in the expiration of the carryforwards before they are utilized.

At December 31, 2019, we recorded a 100% valuation allowance against our deferred tax assets of approximately $245.0 million, as at that time our management believed it was uncertain that they would be fully realized. If we determine in the future that we will be able to realize all or a portion of our net operating loss carryforwards, an adjustment to valuation allowance would increase net income in the period in which we make such a determination.

In general, if we experience a greater than 50 percentage point aggregate change in ownership over a three-year period (a Section 382 ownership change), utilization of our

pre-change

NOL carryforwards are subject to an annual limitation under Section 382 of the Internal Revenue Code (California has similar laws). The annual limitation generally is determined by multiplying the value of our stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term

tax-exempt

rate. Such limitations may result in expiration of a portion of the NOL carryforwards that were generated prior to

2018 before utilization. We have not utilized any NOL carryovers through December 31, 2019.

No liability related to uncertain tax positions is recorded on the consolidated financial statements. All uncertain tax positions are currently recorded as a reduction to our deferred tax asset. It is our policy to include

penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary.

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands):


	December 31,
	2019		2018
Balance at beginning of year	$	16,232	$	11,150
Additions based on tax positions related to current year		5,366		5,144
Decreased for prior period positions		—		(62	)

Unrecognized tax benefit—December 31	$	21,598	$	16,232

We do not expect that our uncertain tax positions will materially change in the next twelve months. The reversal of the uncertain tax benefits will not impact our effective tax rate as we continue to maintain a full valuation allowance against our deferred tax assets.

We file income tax returns in the United States, California and other states. We are not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns will remain open for examination by the federal and state authorities for three and four years, respectively, from the date of utilization of any net operating loss or credits.

Commitments and Contingencies - Additional Information (Detail)		1 Months Ended		12 Months Ended
	Oct. 01, 2019 USD ($)	Aug. 31, 2018 USD ($) ft²	Mar. 31, 2017 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Commitment And Contingencies [Line Items]
Facility size \| ft²		164,150	67,185
Initial term		10 years	10 years
Future minimum rental payments under the Lease term		$ 121,500,000	$ 48,500,000
Lease commencement date			Dec. 15, 2017
Operating lease term		10 years	10 years
Operating lease term, optional extension		10 years	10 years
Repayment of tenant inducement allowance		$ 4,100,000
Contingency losses				$ 0
Provisions for loss contingency				$ 0
Lease term				10 months 3 days
Operating Lease Cost				$ 7,800,000
variable lease cost				2,500,000
Repayemnt of tenant inducement allowance to the landlord			$ 1,700,000
acceleration of depreciation expenses				7,000,000.0
Rent expense				5,200,000	$ 3,600,000	$ 2,000,000.0
Operating lease liabilities	$ 2,900,000			$ 74,225,000
Incremental borrowing rate	8.78%			8.66%
Operating lease right of use asset	$ 0			$ 52,775,000
Gain loss on modification of lease	8,300,000			8,301,000
Unamortized operating lease liability				1,800,000
Tenant inducements receivable				10,700,000
Operating lease payments				8,300,000
Substitute Premises [Member]
Commitment And Contingencies [Line Items]
Operating lease liabilities	53,700,000			72,400,000
Operating lease right of use asset	$ 53,700,000			52,800,000
Tenant inducement allowances received				17,200,000
F. Hoffmann-La Roche Ltd. And Hoffmann-La Roche Inc. [Member]
Commitment And Contingencies [Line Items]
Upfront payments				2,000,000.0
Contingent payment obligations				125,500,000
Contingent Payment Milestone Additional Obligations				5,500,000
F. Hoffmann-La Roche Ltd. And Hoffmann-La Roche Inc. [Member] \| Develpment And Regulatory Milestones [Member]
Commitment And Contingencies [Line Items]
Contingent payment obligations				40,500,000
Contingent Payment Milestone Additional Obligations				19,250,000
F. Hoffmann-La Roche Ltd. And Hoffmann-La Roche Inc. [Member] \| Net Sales Achievement [Member]
Commitment And Contingencies [Line Items]
Contingent payment obligations				85,000,000.0
Syros Pharmaceuticals [Member]
Commitment And Contingencies [Line Items]
Upfront payments				20,000,000.0
Preclinical research payable				40,000,000.0
Payable for exercise of options				$ 315,000,000.0
Maximum [Member]
Commitment And Contingencies [Line Items]
Lease term				10 years
Minimum [Member]
Commitment And Contingencies [Line Items]
Lease term				1 year
Leasehold Improvements [Member]
Commitment And Contingencies [Line Items]
Property palnt and equipment additions				$ 19,000,000.0
Lease Agreements [Member]
Commitment And Contingencies [Line Items]
leasehold improvements Acquired			$ 11,100,000
Lease Agreements [Member] \| Maximum [Member]
Commitment And Contingencies [Line Items]
Tenant inducement		$ 27,900,000

Long-term Debt	12 Months Ended
Long-term Debt	Dec. 31, 2019
Disclosure Text Block [Abstract]
Long-term Debt	7. Long-term Debt Term Loan On December 17, 2019, we entered into the Term Loan for a senior secured credit facility consisting of an initial tra n c he of $75.0 million and the option to draw an additional $75.0 million until December 31, 2020. The first tranche, in the amount of $75.0 million, was funded in connection with the closing date of the Term Loan in December 2019. The Term Loan carries a 72-month term. The Term Loan bears interest at a floating per annum interest rate equal to 7.00% plus the greater of (a) the 3-month LIBOR rate and (b) 2%. In the event we default, the interest rate would be 3% above the rate that is otherwise applicable thereto . Interest on amounts outstanding are payable quarterly in arrears. The Term Loan repayment schedule provides for interest only payments for the first 39 months, followed by consecutive equal quarterly payments of principal and interest commencing in March 2023 and continuing through the maturity of December 2025. We have the option to prepay all or a portion of the borrowed amounts under the Term Loan. If we exercise this option, we must pay a prepayment fee between 1% and 3% of the principal amount being prepaid depending on the timing of the prepayment, or Prepayment Fee. If the prepayment occurs before December 2022, we must also pay an amount equal to the sum of all interest that would have accrued and been payable from date of prepayment through December 2022, or Make Whole Amount. We are obligated to pay an additional fee to the Lenders determined by multiplying the principal amount being paid or prepaid multiplied by 2%, or Paydown Fee , when such payments are made. In the event of default or change in control, all unpaid principal and all accrued and unpaid interest amounts (if any) become immediately due and payable, at which point, we will be subject to the Prepayment Fee, the Make Whole Amount (if any) and the Paydown Fee. Events