Document And Entity Information - shares	9 Months Ended
Document And Entity Information - shares	Sep. 30, 2019	Nov. 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Registrant Name	GOSSAMER BIO, INC.
Trading Symbol	GOSS
Entity Central Index Key	0001728117
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		66,038,122
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity File Number	001-38796
Entity Tax Identification Number	47-5461709
Entity Address, Address Line One	3013 Science Park Road
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Entity Incorporation, State or Country Code	DE
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	684-1300
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 130,708	$ 105,219
Marketable securities	315,764	123,439
Restricted cash		200
Prepaid expenses and other current assets	5,902	3,095
Total current assets	452,374	231,953
Property and equipment, net	5,133	3,193
Operating lease right-of-use assets	10,860
Other assets	1,387	4,273
Total assets	469,754	239,419
Current liabilities
Accounts payable	2,555	2,182
Accrued research and development expenses	19,171	10,653
Accrued expenses	11,443	7,568
Total current liabilities	33,169	20,403
Long-term debt	28,370
Operating lease liabilities	9,342
Accrued expenses - long-term		718
Total liabilities	70,881	21,121
Commitments and contingencies
Stockholders' equity (deficit)
Common stock, $0.0001 par value; 700,000,000 shares authorized as of September 30, 2019 and 49,160,177 shares authorized as of December 31, 2018; 66,038,122 shares issued and 60,984,958 shares outstanding as of September 30, 2019, and 15,533,450 shares issued and 8,051,418 shares outstanding as of December 31, 2018	7	2
Additional paid-in capital	678,010	33,853
Accumulated deficit	(279,472)	(153,863)
Accumulated other comprehensive income (loss)	328	(61)
Total stockholders' equity (deficit)	398,873	(120,069)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	469,754	239,419
Series Seed Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference	0	29,200
Series A Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference	0	79,615
Series B Convertible Preferred Stock
Current liabilities
Convertible preferred stock, shares issued, outstanding, and liquidation preference	$ 0	$ 229,552

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	700,000,000	49,160,177
Common stock, shares, issued	66,038,122	15,533,450
Common stock, shares, outstanding	60,984,958	8,051,418
Series Seed Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	20,000,000
Temporary equity, shares outstanding	0	20,000,000
Temporary equity, liquidation preference	$ 0	$ 20,000
Series A Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	45,714,286
Temporary equity, shares outstanding	0	45,714,286
Temporary equity, liquidation preference	$ 0	$ 80,000
Series B Convertible Preferred Stock
Temporary equity, par value	$ 0.0001	$ 0.0001
Temporary equity, shares issued	0	71,506,513
Temporary equity, shares outstanding	0	71,506,513
Temporary equity, liquidation preference	$ 0	$ 230,000

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 40,148	$ 18,857	$ 100,807	$ 29,411
In process research and development		8,261	2,000	49,659
General and administrative	9,838	22,906	27,544	30,116
Total operating expenses	49,986	50,024	130,351	109,186
Loss from operations	(49,986)	(50,024)	(130,351)	(109,186)
Other income, net	1,486	622	4,742	1,011
Net loss	(48,500)	(49,402)	(125,609)	(108,175)
Other comprehensive income:
Unrealized gain (loss) on marketable securities, net of tax	(168)	232	389	235
Other comprehensive income (loss)	(168)	232	389	235
Comprehensive loss	$ (48,668)	$ (49,170)	$ (125,220)	$ (107,940)
Net loss per share, basic and diluted	$ (0.80)	$ (7.69)	$ (2.39)	$ (17.64)
Weighted average common shares outstanding, basic and diluted	60,755,872	6,423,397	52,535,569	6,133,911

Condensed Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in-Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Series Seed Convertible Preferred Stock	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock
Beginning balance at Dec. 31, 2017	$ (6,862)	$ 0	$ 32	$ (6,894)	$ 0
Temporary equity, beginning balance, shares at Dec. 31, 2017						0	0	0
Temporary equity, beginning balance at Dec. 31, 2017						$ 0	$ 0	$ 0
Beginning balance, shares at Dec. 31, 2017		9,160,888
Issuance of Series A preferred stock for cash, net of $0.4 million in offering costs							$ 71,944
Issuance of Series A preferred stock for cash, net of $0.4 million in offering costs, shares							41,328,286
Temporary equity, issuance of stock for acquisition						$ 29,200
Temporary equity, issuance of stock for acquisition, shares						20,000,000
Issuance of stock for acquisition	2,875	$ 1	2,874
Issuance of stock for acquisition. shares		1,101,278
Issuance of Series A preferred stock to convert debt and accrued interest							$ 6,124
Issuance of Series A preferred stock to convert debt and accrued interest, shares							3,499,209
Stock-based compensation	605		605		0
Incremental vesting conditions place on previously issued common shares		(4,580,444)
Net loss	(26,037)			(26,037)	0
Ending balance at Mar. 31, 2018	(29,419)	$ 1	3,511	(32,931)	0
Temporary equity, ending balance, shares at Mar. 31, 2018						20,000,000	44,827,495	0
Temporary equity, ending balance at Mar. 31, 2018						$ 29,200	$ 78,068	$ 0
Ending balance, shares at Mar. 31, 2018		5,681,722
Beginning balance at Dec. 31, 2017	(6,862)	$ 0	32	(6,894)	0
Temporary equity, beginning balance, shares at Dec. 31, 2017						0	0	0
Temporary equity, beginning balance at Dec. 31, 2017						$ 0	$ 0	$ 0
Beginning balance, shares at Dec. 31, 2017		9,160,888
Net loss	(108,175)
Other comprehensive income (loss)	235
Ending balance at Sep. 30, 2018	(91,778)	$ 2	23,054	(115,069)	235
Temporary equity, ending balance, shares at Sep. 30, 2018						20,000,000	45,714,286	71,506,513
Temporary equity, ending balance at Sep. 30, 2018						$ 29,200	$ 79,615	$ 229,552
Ending balance, shares at Sep. 30, 2018		7,733,845
Beginning balance at Mar. 31, 2018	(29,419)	$ 1	3,511	(32,931)	0
Temporary equity, beginning balance, shares at Mar. 31, 2018						20,000,000	44,827,495	0
Temporary equity, beginning balance at Mar. 31, 2018						$ 29,200	$ 78,068	$ 0
Beginning balance, shares at Mar. 31, 2018		5,681,722
Issuance of Series A preferred stock to convert debt and accrued interest, shares							886,791
Issuance of Series A preferred stock to convert debt and accrued interest	0						$ 1,547
Vesting of restricted stock	0
Vesting of restricted stock, shares		251,542
Stock-based compensation	1,374		1,374
Stock-based compensation, shares		0
Net loss	(32,736)			(32,736)
Other comprehensive income (loss)	3				3
Ending balance at Jun. 30, 2018	(60,778)	$ 1	4,885	(65,667)	3
Temporary equity, ending balance, shares at Jun. 30, 2018						20,000,000	45,714,286	0
Temporary equity, ending balance at Jun. 30, 2018						$ 29,200	$ 79,615	$ 0
Ending balance, shares at Jun. 30, 2018		5,933,264
Issuance of Series B preferred stock for cash, net of $0.5 in offering costs	0							$ 229,552
Issuance of Series B preferred stock for cash, net of $0.5 in offering costs, shares								71,506,513
Vesting of restricted stock	1	$ 1
Vesting of restricted stock, shares		1,800,581
Stock-based compensation	18,169		18,169
Stock-based compensation, shares		0
Net loss	(49,402)			(49,402)
Other comprehensive income (loss)	232				232
Ending balance at Sep. 30, 2018	(91,778)	$ 2	23,054	(115,069)	235
Temporary equity, ending balance, shares at Sep. 30, 2018						20,000,000	45,714,286	71,506,513
Temporary equity, ending balance at Sep. 30, 2018						$ 29,200	$ 79,615	$ 229,552
Ending balance, shares at Sep. 30, 2018		7,733,845
Beginning balance at Dec. 31, 2018	(120,069)	$ 2	33,853	(153,863)	(61)
Temporary equity, beginning balance, shares at Dec. 31, 2018						20,000,000	45,714,286	71,506,513
Temporary equity, beginning balance at Dec. 31, 2018						$ 29,200	$ 79,615	$ 229,552
Beginning balance, shares at Dec. 31, 2018		8,051,418
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs	291,344	$ 2	291,342
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs, shares		19,837,500
Conversion of convertible preferred stock into common stock	338,367	$ 3	338,364
Convertible preferred stock, conversion of convertible preferred stock into common stock, shares						(20,000,000)	(45,714,286)	(71,506,513)
Convertible preferred stock, conversion of convertible preferred stock into common stock						$ (29,200)	$ (79,615)	$ (229,552)
Conversion of convertible preferred stock into common stock, shares		30,493,460
Vesting of restricted stock	0
Vesting of restricted stock, shares		1,619,592
Stock-based compensation	3,089		3,089
Stock-based compensation, shares		27,500
Net loss	(32,611)			(32,611)
Other comprehensive income (loss)	140				140
Ending balance at Mar. 31, 2019	480,260	$ 7	666,648	(186,474)	79
Temporary equity, ending balance, shares at Mar. 31, 2019						0	0	0
Temporary equity, ending balance at Mar. 31, 2019						$ 0	$ 0	$ 0
Ending balance, shares at Mar. 31, 2019		60,029,470
Beginning balance at Dec. 31, 2018	$ (120,069)	$ 2	33,853	(153,863)	(61)
Temporary equity, beginning balance, shares at Dec. 31, 2018						20,000,000	45,714,286	71,506,513
Temporary equity, beginning balance at Dec. 31, 2018						$ 29,200	$ 79,615	$ 229,552
Beginning balance, shares at Dec. 31, 2018		8,051,418
Conversion of convertible preferred stock into common stock, shares		30,493,460
Exercise of stock options, shares	173,712
Net loss	$ (125,609)
Other comprehensive income (loss)	389
Ending balance at Sep. 30, 2019	398,873	$ 7	678,010	(279,472)	328
Temporary equity, ending balance, shares at Sep. 30, 2019						0	0	0
Temporary equity, ending balance at Sep. 30, 2019						$ 0	$ 0	$ 0
Ending balance, shares at Sep. 30, 2019		60,984,958
Beginning balance at Mar. 31, 2019	480,260	$ 7	666,648	(186,474)	79
Temporary equity, beginning balance, shares at Mar. 31, 2019						0	0	0
Temporary equity, beginning balance at Mar. 31, 2019						$ 0	$ 0	$ 0
Beginning balance, shares at Mar. 31, 2019		60,029,470
Exercise of stock options	86		86
Exercise of stock options, shares		33,273
Vesting of restricted stock, shares		404,637
Stock-based compensation	5,140		5,140
Other additional paid-in capital	39		39
Net loss	(44,498)			(44,498)
Other comprehensive income (loss)	417				417
Ending balance at Jun. 30, 2019	441,444	$ 7	671,913	(230,972)	496
Temporary equity, ending balance, shares at Jun. 30, 2019						0	0	0
Temporary equity, ending balance at Jun. 30, 2019						$ 0	$ 0	$ 0
Ending balance, shares at Jun. 30, 2019		60,467,380
Exercise of stock options	369		369
Exercise of stock options, shares		112,939
Vesting of restricted stock, shares		404,639
Stock-based compensation	5,728		5,728
Net loss	(48,500)			(48,500)
Other comprehensive income (loss)	(168)				(168)
Ending balance at Sep. 30, 2019	$ 398,873	$ 7	$ 678,010	$ (279,472)	$ 328
Temporary equity, ending balance, shares at Sep. 30, 2019						0	0	0
Temporary equity, ending balance at Sep. 30, 2019						$ 0	$ 0	$ 0
Ending balance, shares at Sep. 30, 2019		60,984,958

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (125,609)	$ (108,175)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	625	163
Stock-based compensation expense	13,957	20,148
In process research and development expenses	2,000	49,659
Amortization of long-term debt discount and issuance costs	148
Amortization of premium on investments, net of accretion of discounts	(2,296)
Net realized gain on investments	(1)
Changes in operating assets and liabilities:
Operating lease right of use assets and liabilities, net	49
Prepaid expenses and other current assets	(2,807)	(1,442)
Other assets	2,886	(823)
Accounts payable	333	3,525
Accrued expenses	(760)	3,263
Accrued research and development expenses	8,518	7,778
Accrued compensation and benefits	2,263
Accrued interest expense		(117)
Net cash used in operating activities	(100,694)	(26,021)
Cash flows from investing activities
Research and development asset acquisitions, net of cash acquired	(2,000)	(17,721)
Purchase of investments	(399,393)	(115,141)
Maturities of investments	205,750
Sales of investments	4,004
Purchase of property and equipment	(2,438)	(3,168)
Net cash used in investing activities	(194,077)	(136,030)
Cash flows from financing activities
Proceeds from issuance of common stock in a public offering, net	291,311
Proceeds from the issuance of long-term debt, net of issuance costs of $1,778	28,222
Proceeds from the exercise of stock options	527
Repayment of notes payable to related parties		(40)
Net cash provided by financing activities	320,060	303,003
Net increase in cash, cash equivalents and restricted cash	25,289	140,952
Cash, cash equivalents and restricted cash, at the beginning of the period	105,419	315
Cash, cash equivalents and restricted cash, at the end of the period	130,708	141,267
Supplemental disclosure of cash flow information:
Cash paid for interest	854
Supplemental disclosure of noncash investing and financing activities:
Acquisition of in-process research and development through issuance of stock		19,284
Issuance of Series A convertible preferred stock to convert debt and accrued interest		6,124
Recognition of operating lease right of use asset	12,458
Recognition of operating lease liabilities	13,182
Conversion of convertible preferred stock to common stock	338,367
Change in unrealized gain on marketable securities, net of tax	510	235
Change in unrealized loss on foreign currency translations, net of tax	121
Unpaid property and equipment	$ 127
Series A Convertible Preferred Stock
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net		73,491
Series B Convertible Preferred Stock
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net		$ 229,552

Description of the Business

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Description of the Business

1. Description of the Business

Gossamer Bio, Inc. (including its subsidiaries, referred to as “we,” “us,” “our,”, or the “Company”) is a clinical-stage biopharmaceutical company focused on discovering, acquiring, developing and commercializing therapeutics in the disease areas of immunology, inflammation and oncology. The Company was incorporated in the state of Delaware on October 25, 2015 (originally as FSG Bio, Inc.) and is based in San Diego, California.

The condensed consolidated financial statements include the accounts of Gossamer Bio, Inc. and its wholly owned subsidiaries. All intercompany balances and transactions among the consolidated entity have been eliminated in consolidation.

Initial Public Offering in February 2019

On February 12, 2019, the Company completed its initial public offering (“IPO”) with the sale of 19,837,500 shares of common stock, including shares of common stock issued upon the exercise in full of the underwriters’ option to purchase additional shares, at a public offering price of $16.00 per share, resulting in net proceeds of $291.3 million, after deducting underwriting discounts, commissions, and offering expenses.

In addition, in connection with the completion of the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 30,493,460 shares of common stock.

Liquidity and Capital Resources

The Company has incurred significant operating losses since its inception. As of September 30, 2019, the Company had an accumulated deficit of $279.5 million. From the Company’s inception through September 30, 2019, the Company has funded its operations primarily through equity financings, including the Company’s IPO which closed on February 12, 2019. The Company raised $601.3 million from October 2017 through March 2019 through Series A and Series B Convertible Preferred Stock financings, a convertible note financing, and the IPO, after deducting underwriting discounts, commissions, and offering expenses. In addition, the Company received $12.8 million in cash in connection with the January 2018 acquisition of AA Biopharma Inc. On May 2, 2019 the Company, as guarantor, and its wholly-owned subsidiary GB001, Inc., as borrower, entered into a credit, guaranty and security agreement, as amended on September 18, 2019, (the “Credit Facility”) with MidCap Financial Trust (“MidCap”), an agent and as a lender, and the additional lenders party thereto from time to time (together with MidCap, the “Lenders”), pursuant to which the Lenders, including affiliates of MidCap and Silicon Valley Bank agreed to make term loans available to the Company for working capital and general business purposes, in a principal amount of up to $150.0 million in term loan commitments, including a $30.0 million term loan that was funded at the closing date. Under the Credit Facility, the Company has the ability to access the remaining $120.0 million in three additional tranches (of $40.0 million, $30.0 million and $50.0 million, respectively), subject to specified availability periods, the achievement of certain clinical development milestones, minimum cash requirements and other customary conditions. As of September 30, 2019, no other tranches under the Credit Facility have been drawn. See Note 5 for additional information regarding the Credit Facility.

The Company expects to continue to incur significant operating losses for the foreseeable future and may never become profitable. As a result, the Company will need to raise capital through equity offerings, debt financings other capital sources, including potential collaborations, licenses and other similar arrangements. Management believes that it has sufficient working capital on hand to fund operations through at least the next twelve months from the date these condensed consolidated financial statements were available to be issued. There can be no assurance that the Company will be successful in acquiring additional funding, that the Company’s projections of its future working capital needs will prove accurate, or that any additional funding would be sufficient to continue operations in future years.

Subsequent Events

The Company has evaluated subsequent events through November 12, 2019, the issuance date of the condensed consolidated financial statements, and has determined that there were no material subsequent events to recognize or disclose.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (“GAAP”) for interim financial information and with the instructions of the Securities and Exchange Commission (“SEC”) on Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for complete financial statements. In the opinion of management, the condensed consolidated financial statements include all adjustments necessary, which are of a normal and recurring nature, for the fair presentation of the Company’s financial position and of the results of operations and cash flows for the periods presented. These financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto for the year ended December 31, 2018 included in the Company’s Annual Report on Form 10-K filed with the SEC on March 22, 2019. The results of operations for the interim period shown in this report are not necessarily indicative of the results that may be expected for any other interim period or for the full year. The balance sheet at December 31, 2018, has been derived from the audited financial statements at that date.

Use of Estimates

The preparation of condensed consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. The most significant estimates in the Company’s condensed consolidated financial statements relate to accrued research and development expenses, the valuation of preferred and common stock, the valuation of stock options and the valuation allowance of deferred tax assets resulting from net operating losses. These estimates and assumptions are based on current facts, historical experience and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results could differ from those estimates.

Recently Adopted Accounting Pronouncements

The Company adopted Accounting Standards Update (“ASU”) No. 2016-02, Leases (Topic 842) (commonly referred to as Accounting Standards Codification (“ASC”) 842), as of January 1, 2019, using the optional transition method. The optional transition method provides a method for recording existing leases at adoption and a cumulative catch up adjustment on January 1, 2019 for any differences between ASC 842 and the legacy guidance provided in ASC 840, Leases that would have impacted our income statement. No retrospective restatements are required under the optional transition method. In addition, the Company elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allowed us to carry forward the historical lease classification. The Company also applied the short-term lease recognition exemption for leases with terms at inception not greater than 12 months.

Adoption of the new standard resulted in the recording of additional operating lease right-of-use assets and operating lease liabilities of approximately $12.5 million and $13.2 million, respectively, as of January 1, 2019. The difference between the operating lease right-of-use assets and lease liabilities are due to accrued deferred rent and unamortized lease incentives.

Recently Issued Accounting Pronouncements – Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments, which changes the impairment model for most financial assets and certain other instruments. For trade receivables and other instruments, entities will be required to use a new forward-looking expected loss model that generally will result in the earlier recognition of allowances for losses. For available-for-sale debt securities with unrealized losses, the losses will be recognized as allowances rather than as reductions in the amortized cost of the securities. This guidance is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those years, with early adoption permitted only as of annual reporting periods beginning after December 15, 2018. We are currently evaluating the timing and impact of the adoption of ASU 2016-13 on our unaudited condensed financial statements or related financial statement disclosures.

Net Loss Per Share

Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss per share excludes the potential impact of the Company’s Series Seed Convertible Preferred Stock, Series A Convertible Preferred Stock, and Series B Convertible Preferred Stock, common stock options and unvested shares of restricted stock because their effect would be anti-dilutive due to the Company’s net loss. Since the Company had a net loss in each of the periods presented, basic and diluted net loss per common share are the same.

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of September 30,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		10,158,710
Shares issuable upon conversion of Series B Convertible Preferred Stock		—		15,890,306
Shares issuable upon exercise of stock options		8,099,861		2,104,311
Non-vested shares under restricted stock grants		5,053,169		7,799,605

Balance Sheet Accounts and Supplemental Disclosures

9 Months Ended

Sep. 30, 2019

Balance Sheets Accounts And Supplemental Disclosures [Abstract]

Balance Sheet Accounts and Supplemental Disclosures

3. Balance Sheet Accounts and Supplemental Disclosures

Property and Equipment

Property and equipment, net consisted of the following (in thousands):

	Estimated Useful Life (in years)	September 30, 2019		December 31, 2018
Office equipment	3-7	$	1,097	$	918
Computer equipment	5		112		15
Software	3		78		50
Lab equipment	2-5		2,530		1,070
Leasehold improvements	6-7		2,170		1,243
Construction in process	N/A		68		194
Total property and equipment			6,055		3,490
Less: accumulated depreciation			922		297
Property and equipment, net		$	5,133	$	3,193

Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	As of
	September 30, 2019		December 31, 2018
Accrued compensation	$	6,365	$	4,102
Operating lease liabilities		2,291		—
Accrued professional service fees		2,059		2,697
Accrued other		728		769
Total accrued expenses	$	11,443	$	7,568

Fair Value Measurements and Available for Sale Investments

9 Months Ended

Sep. 30, 2019

Fair Value Measurements And Available For Sale Investments [Abstract]

Fair Value Measurements and Available for Sale Investments

4. Fair Value Measurements and Available for Sale Investments

Fair Value Measurements

The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

Level 1: Observable inputs such as quoted prices in active markets;

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

We classify our cash equivalents and available-for-sale investments within Level 1 or Level 2. The fair value of our investment grade corporate debt securities and commercial paper is determined using proprietary valuation models and analytical tools, which utilize market pricing or prices for similar instruments that are both objective and publicly available, such as matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, and offers.

Assets and Liabilities Measured at Fair Value on a Recurring Basis

The following table presents the hierarchy for assets measured at fair value on a recurring basis as of September 30, 2019 and December 31, 2018 (in thousands):

	Fair Value Measurements at End of Period Using:
				Quoted Market		Significant		Significant
				Prices for		Other Observable		Unobservable
		Total		Identical Assets		Inputs		Inputs
		Fair Value		(Level 1)		(Level 2)		(Level 3)
As of September 30, 2019
Money market funds		$	35,756	$	35,756	$	—	$	—
U.S. Treasury and agency securities			199,500		199,500		—		—
Commercial paper			30,985		—		30,985		—
Corporate debt securities			99,892		—		99,892		—
As of December 31, 2018
Money market funds		$	17,295	$	17,295	$	—	$	—
U.S. Treasury securities			123,439		123,439		—		—

The Company did not reclassify any investments between levels in the fair value hierarchy during the periods presented.

Fair Value of Other Financial Instruments

As of September 30, 2019 and December 31, 2018, the carrying amounts of the Company’s financial instruments, which include cash, interest and securities receivable, accounts payable and accrued expenses, approximate fair values because of their short maturities.

Interest and securities receivable as of September 30, 2019 and December 31, 2018, was $1.2 million and $0.6 million, respectively, and is recorded as a component of prepaid expenses and other current assets on the condensed consolidated balance sheets. Securities receivable reflect the timing differences of maturities or settlements of investments and the ultimate reinvestment of such amounts.

We believe that our term loan facility bears interest at a rate that approximates prevailing market rates for instruments with similar characteristics and, accordingly, the carrying value of the term loan facility approximates fair value. We estimate the fair value of long-term debt utilizing an income approach. We use a present value calculation to discount principal and interest payments and the final maturity payment on these liabilities using a discounted cash flow model based on observable inputs. The debt instrument is then discounted based on what the current market rates would be as of the reporting date. Based on the assumptions used to value these liabilities at fair value, the debt instrument is categorized as Level 2 in the fair value hierarchy.

Available for Sale Investments

We invest our excess cash in U.S. Treasury and agency securities and debt instruments of corporations and commercial obligations, which are classified as available-for-sale investments. These investments are carried at fair value and are included in the tables above. The Company evaluates securities with unrealized losses to determine whether such losses, if any, are other than temporary. Realized gains and losses are calculated using the specific identification method and recorded as interest income or expense. We do not generally intend to sell the investments and it is not more likely than not that we will be required to sell the investments before recovery of their amortized cost bases, which may be at maturity.

The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in marketable securities and long-term investments as of September 30, 2019 are as follows (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury and agency securities	$	185,844	$	145	$	(4	)	$	185,985
Commercial paper		29,887		—		—			29,887
Corporate debt securities		99,578		319		(5	)		99,892
Total marketable securities	$	315,309	$	464	$	(9	)	$	315,764

As of September 30, 2019, the Company classified $14.6 million of assets with original maturities of 90 days or less as cash equivalents. None of the investments have been in a gross unrealized loss for a period greater than 12 months. At each reporting date, we perform an evaluation of impairment to determine if any unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of the amortized cost basis. We intend and have the ability to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Further, based on our evaluation, we determined that unrealized losses were not other-than-temporary as of September 30, 2019.

Contractual maturities of available-for-sale debt securities, as of September 30, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	290,813
One to two years		24,951
Total	$	315,764

We have the ability, if necessary, to liquidate any of our cash equivalents and marketable securities to meet our liquidity needs in the next 12 months. Accordingly, those investments with contractual maturities greater than one year from the date of purchase are classified as current assets on the accompanying condensed consolidated balance sheets.

Long-Term Debt

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Long-term Debt

5. Long-term Debt

On May 2, 2019, the Company, as guarantor, and its wholly-owned subsidiary GB001, Inc., as borrower, entered into the Credit Facility described in Note 1, pursuant to which the Lenders, including affiliates of MidCap and Silicon Valley Bank, agreed to make term loans available to the Company for working capital and general business purposes, in a principal amount of up to $150.0 million in term loan commitments, including a $30.0 million term loan that was funded at the closing date, with the ability to access the remaining $120.0 million in three additional tranches (of $40.0 million, $30.0 million and $50.0 million, respectively), subject to specified availability periods, the achievement of certain clinical development milestones, minimum cash requirements and other customary conditions. The second tranche is available no earlier than February 1, 2020 and no later than July 31, 2020. The third tranche is available no earlier than May 1, 2020 and no later than October 31, 2020. The fourth tranche is available no earlier than February 1, 2021 and no later than July 31, 2021. The Credit Facility is secured by substantially all of the Company’s and its domestic subsidiaries’ personal property, including intellectual property, and includes affirmative and negative covenants applicable to the Company.

Each term loan under the Credit Facility bears interest at an annual rate equal to the sum of (i) one-month LIBOR (customarily defined, with a change to prime rate if LIBOR funding becomes unlawful or impractical) plus (ii) 6.15%, subject to a LIBOR floor of 2.00%. The borrower is required to make interest-only payments on the term loan for all payment dates prior to June 1, 2021. The term loans under the Credit Facility will begin amortizing on June 1, 2021, with equal monthly payments of principal plus interest being made by the Company to the Lenders in consecutive monthly installments following such interest-only period for 36 months or, for any funding of the fourth tranche occurring after June 1, 2021, the number of months until the Credit Facility matures on May 1, 2024. Upon final repayment of the term loans, the borrower must pay an exit fee of 1.75% of the amount borrowed under the Credit Facility, less any partial exit fees previously paid. Upon partial prepayment of a portion of the term loans, the borrower must pay a partial exit fee of 1.75% of the principal being prepaid. At the borrower’s option, the borrower may prepay the outstanding principal balance of the term loan in whole or in part, subject to a prepayment fee of 3.0% of any amount prepaid if the prepayment occurs through and including the first anniversary of the closing date, 2.0% of the amount prepaid if the prepayment occurs after the first anniversary of the closing date through and including the second anniversary of the closing date, and 1.0% of any amount prepaid after the second anniversary of the closing date and prior to May 1, 2024.

The Credit Facility includes affirmative and negative covenants applicable to the Company and certain of its subsidiaries. The affirmative covenants include, among others, covenants requiring such entities to maintain their legal existence and governmental approvals, deliver certain financial reports, maintain insurance coverage, maintain property, pay taxes, satisfy certain requirements regarding accounts and comply with laws and regulations. The negative covenants include, among others, restrictions on such entities from transferring collateral, incurring additional indebtedness, engaging in mergers or acquisitions, paying dividends or making other distributions, making investments, creating liens, amending material agreements and organizational documents, selling assets and suffering a change in control, in each case subject to certain exceptions. The Company and certain of its subsidiaries are also subject to an ongoing minimum cash financial covenant in which they must maintain unrestricted cash in an amount not less than 25% of the outstanding principal amount of the term loans. As of September 30, 2019, the Company was in compliance with these covenants.

The Credit Facility also includes events of default, the occurrence and continuation of which could cause interest to be charged at the rate that is otherwise applicable plus 3.0% and would provide MidCap, as agent, with the right to exercise remedies against the Company and/or certain of its subsidiaries, and the collateral securing the Credit Facility, including foreclosure against the properties securing the credit facilities, including cash. These events of default include, among other things, failure to pay any amounts due under the Credit Facility, a breach of covenants under the Credit Facility, insolvency or the occurrence of insolvency events, the occurrence of a change in control, the occurrence of certain U.S. Food and Drug Administration and regulatory events, failure to remain registered with the SEC and listed for trading on NASDAQ, the occurrence of a material adverse change, the occurrence of a default under a material agreement reasonably expected to result in a material adverse change, the occurrence of certain defaults under certain other indebtedness in an amount greater than $2,500,000 and the occurrence of certain defaults under subordinated indebtedness and convertible indebtedness.

Our long-term debt as of September 30, 2019 consisted of the following (in thousands):

	September 30, 2019
Term loan	$	30,000
Debt discount and issuance costs		(1,630	)
Long-term debt	$	28,370

The scheduled future minimum principal payments are as follows (in thousands)

	September 30, 2019
2019 (three-months)	$	—
2020		—
2021		5,833
2022		10,000
2023		10,000
2024		4,167
Total	$	30,000

Asset Acquisitions and Contingent Consideration

9 Months Ended

Sep. 30, 2019

Business Combinations [Abstract]

Asset Acquisitions and Contingent Consideration

7. Asset Acquisitions and Contingent Consideration

The following purchased assets were accounted for as asset acquisitions as substantially all of the fair value of the assets acquired were concentrated in a group of similar assets and/or the acquired assets were not capable of producing outputs due to the lack of employees and early stage of development. Because the assets had not yet received regulatory approval, the fair value attributable to these assets was recorded as in process research and development (“IPR&D”) expenses in the Company’s condensed consolidated statement of operations for the three and nine months ended September 30, 2019.

The Company accounts for contingent consideration payable upon achievement of certain regulatory, development or sales milestones in such asset acquisitions when the underlying contingency is met.

Acquisition of License from Pulmokine, Inc. (GB002)

On October 2, 2017, the Company, entered into a license agreement with Pulmokine, Inc. under which it was granted an exclusive worldwide license and sublicense to certain intellectual property rights owned or controlled by Pulmokine to develop and commercialize GB002 and certain backup compounds for the treatment, prevention and diagnosis of any and all disease or conditions. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The assets acquired are in the early stages of the U.S. Food and Drug Administration (“FDA”) approval process, and the Company intends to further develop the assets acquired through potential FDA approval as evidenced by the milestone arrangement in the contract. The development activities cannot be performed without significant cost and effort by the Company. The agreement will remain in effect from the effective date, unless terminated earlier, until, on a licensed product-by-licensed product and country-by-country basis, the later of ten years from the date of first commercial sale or when there is no longer a valid patent claim covering such licensed product or specified regulatory exclusivity for the licensed product in such country. The Company is obligated to make future development and regulatory milestone payments of up to $63.0 million, commercial milestone payments of up to $45.0 million, and sales milestone payments of up to $190.0 million. The Company is also obligated to pay tiered royalties on sales for each licensed product, at percentages ranging from the mid-single digits to the high single-digits. The Company made an upfront payment of $5.5 million in October 2017. As of September 30, 2019, no milestones had been accrued as the underlying contingencies had not yet been met.

AA Biopharma Inc. Acquisition (GB001)

On January 4, 2018, the Company acquired AA Biopharma Inc. pursuant to a merger agreement, and with the acquisition acquired the rights to GB001 and certain backup compounds. In connection with the merger agreement, the Company issued an aggregate of 20,000,000 shares of Series Seed Convertible Preferred Stock and 1,101,278 shares of Common Stock to the AA Biopharma shareholders. The Company recorded IPR&D of $19.3 million in January 2018 in connection with the acquisition of AA Biopharma.

Acquisition of License from Aerpio Pharmaceuticals, Inc. (GB004)

On June 24, 2018, the Company entered into a license agreement with Aerpio Pharmaceuticals, Inc. (“Aerpio”) under which the Company was granted an exclusive worldwide license and sublicense to certain intellectual property rights owned or controlled by Aerpio to develop and commercialize GB004, and certain other related compounds for all applications. The Company also has the right to sublicense its rights under the license agreement, subject to certain conditions. The Company is obligated to make future development and regulatory milestone payments of up to $55.0 million, commercial milestone payments of up to $85.0 million and sales milestone payments of up to $260.0 million. The Company is also obligated to pay tiered royalties on sales for each licensed product, at percentages ranging from a high single-digit to mid-teens, subject to certain customary reductions. The Company made an upfront payment of $20.0 million in June 2018, which represented the purchase consideration for an asset acquisition. As of September 30, 2019, no milestones had been accrued as the underlying contingencies had not yet been met.

Adhaere Pharmaceuticals, Inc. Acquisition (GB1275)

On September 21, 2018, the Company acquired Adhaere Pharmaceuticals, Inc. (“Adhaere”) pursuant to a merger agreement for an upfront payment of $7.5 million in cash, and with the acquisition acquired the rights to GB1275 and certain backup compounds. The Company is obligated to make regulatory, development and sales milestone payments of up to $62.0 million and pay tiered royalties on worldwide net sales, at percentages ranging from low to mid-single digits, subject to customary reductions. In September 2018, the Company recorded IPR&D of $7.5 million in connection with the acquisition of Adhaere. In May 2019, the Company made a milestone payment of $1.0 million in connection with the filing of the Investigational New Drug (IND) application for the GB1275 program. As of September 30, 2019, no other milestones had been accrued as the underlying contingencies had not yet been met.

The Company recorded the following IPR&D expense on the condensed consolidated statements of operations (in thousands):

	Three months ended September 30,				Nine months ended September 30,
	2019		2018		2019		2018
GB001	$	—	$	—	$	—	$	19,148
GB004		—		—		—		20,000
GB1275		—		7,500		1,000		7,500
Other Programs		—		761		1,000		3,011
Total in process research and development	$	—	$	8,261	$	2,000	$	49,659

Stockholders' Equity (Deficit)

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

Stockholders' Equity (Deficit)

8. Stockholders’ Equity (Deficit)

In connection with the Company’s IPO, the outstanding shares of the Company’s Series Seed, Series A, and Series B Convertible Preferred Stock automatically converted into 30,493,460 shares of common stock.

Common Stock

On December 3, 2015, the Company issued 9,160,888 shares of common stock as founder shares for services rendered to the Company, valued at $0.0001 par value per share, for a total of approximately $4,100. On January 4, 2018, incremental vesting conditions were placed on the previously issued founder shares. Fifty percent of the previously issued founder shares vested on January 4, 2018, and the remaining founder shares are subject to vesting restrictions over a period of five years.

Pursuant to the employment agreements with the Company’s founders executed January 4, 2018, the Company provided for certain potential additional issuances of common stock (the “anti-dilution shares”) to each of the founders to ensure the total number of shares of common stock held by them and their affiliates (inclusive of any shares subject to equity awards granted by the Company) would represent 15% of the Company’s fully-diluted capitalization until such time as the Company raised $300 million in equity capital, including the capital raised in the Series A financing.

In furtherance of this obligation, on May 21, 2018, the Company issued 251,547 shares of common stock to the founders for services rendered to the Company, valued at $2.61 per share with an additional 251,547 shares of restricted stock subject to the same vesting restrictions and vesting period as the founder shares. In addition, on September 6, 2018, the Company issued 1,795,023 shares of common stock to the founders for services rendered to the Company, valued at $9.63 per share, with an additional 1,795,023 shares of restricted stock subject to the same vesting restrictions and vesting period as the founder shares.

Each share of common stock is entitled to one vote. Common stock owners are entitled to dividends when funds are legally available and declared by the Board.

Shares of Common Stock Subject to Repurchase

In November 2017, in connection with the issuance of the Series A Convertible Preferred Stock, certain employees entered into stock restriction agreements, whereby 1,305,427 shares are subject to forfeiture by the Company upon the stockholder’s termination of employment or service to the Company. In January 2018, the Company’s founders entered into stock restriction agreements, whereby 4,580,444 of previously unrestricted shares of common stock were subject to service vesting conditions. These shares are also subject to forfeiture by the Company upon the stockholders’ termination of employment or service to the Company. Any shares subject to repurchase by the Company are not deemed, for accounting purposes, to be outstanding until those shares vest. As such, the Company recognizes the measurement date fair value of the restricted stock over the vesting period as compensation expense. As of September 30, 2019 and December 31, 2018, 5,053,169 and 7,482,032 shares of common stock were subject to repurchase by the Company, respectively. The unvested stock liability related to these awards is immaterial to all periods presented.

Equity Incentive Plans

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

9. Equity Incentive Plans

Approval of the 2019 Equity Incentive Plan

In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Incentive Award Plan (the “2019 Plan”). The 2019 Plan became effective on February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. Under the 2019 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units, and other stock or cash-based awards to individuals who are then employees, officers, directors or consultants of the Company, and employees and consultants of the Company’s subsidiaries. A total of 5,750,000 shares of common stock were approved to be initially reserved for issuance under the 2019 Plan. The number of shares that remained available for issuance under the 2017 Plan (as defined below) as of the effective date of the 2019 Plan were, and shares subject to outstanding awards under the 2017 Plan as of the effective date of the 2019 Plan that are subsequently canceled, forfeited or repurchased by the Company will be added to the shares reserved under the 2019 Plan. In addition, the number of shares of common stock available for issuance under the 2019 Plan will be automatically increased on the first day of each calendar year during the ten-year term of the 2019 Plan, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 5% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of September 30, 2019, an aggregate of 2,486,308 shares of common stock were available for issuance under the 2019 Plan and 3,263,692 shares of common stock were subject to outstanding awards under the 2019 Plan.

Approval of the 2019 Employee Stock Purchase Plan

In January 2019, the Company’s board of directors and stockholders approved and adopted the 2019 Employee Stock Purchase Plan (the “ESPP”). The ESPP became effective as of February 6, 2019, the day prior to the effectiveness of the registration statement filed in connection with the IPO. The ESPP permits participants to purchase common stock through payroll deductions of up to 20% of their eligible compensation. A total of 700,000 shares of common stock were approved to be initially reserved for issuance under the ESPP. In addition, the number of shares of common stock available for issuance under the ESPP will be automatically increased on the first day of each calendar year during the first ten-years of the term of the ESPP, beginning with January 1, 2020 and ending with January 1, 2029, by an amount equal to 1% of the outstanding number of shares of the Company’s common stock on December 31 of the preceding calendar year or such lesser amount as determined by the Company’s board of directors. As of September 30, 2019, an aggregate of 700,000 shares of common stock were available for issuance under the ESPP.

2017 Equity Incentive Plan

The Company’s 2017 Equity Incentive Plan (the “2017 Plan”) permitted the granting of incentive stock options, non-statutory stock options, restricted stock, restricted stock units and other stock-based awards. Subsequent to the adoption of the 2019 Plan, no additional equity awards can be made under the 2017 Plan. As of September 30, 2019, 4,819,318 shares of common stock were subject to outstanding options under the 2017 Plan, and 635,155 shares of restricted stock awards granted under the 2017 plan were unvested.

Stock Options

The fair value of each employee and non-employee stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company, prior to its IPO on February 12, 2019, was a private company and lacks company-specific historical and implied volatility information. Therefore, it estimates its expected volatility based on the historical volatility of a publicly traded set of peer companies. Due to the lack of historical exercise history, the expected term of the Company’s stock options for employees has been determined utilizing the “simplified” method for awards. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is zero based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future.

The following table summarizes stock option activity during the nine months ended September 30, 2019:

	Shares Subject to Options Outstanding					Weighted- Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Outstanding as of December 31, 2018		5,107,329		$	7.51		9.7	$	16,343
Options granted		3,441,901		$	20.79
Option exercised		(173,712	)	$	3.04
Options forfeited/cancelled		(275,657	)	$	12.30
Outstanding as of September 30, 2019		8,099,861		$	13.09		9.2	$	44,015
Options vested and exercisable as of September 30, 2019		644,722		$	3.88		8.7	$	8,323

The aggregate intrinsic value in the above table is calculated as the difference between fair value of the Company’s common stock price on September 30, 2019 and the exercise price of the stock options. The weighted-average grant date fair value per share for the stock option grants during the nine months ended September 30, 2019 was $20.79. At September 30, 2019, the total unrecognized compensation related to unvested stock option awards granted was $57.8 million, which the Company expects to recognize over a weighted-average period of approximately 3.0 years.

Restricted Stock

The summary of the Company’s restricted stock activity is as follows:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Nonvested at December 31, 2018		7,482,032		$	4.01
Granted		—			—
Vested		(2,428,863	)	$	4.11
Forfeited		—			—
Nonvested at September 30, 2019		5,053,169		$	3.96

At September 30, 2019, the total unrecognized compensation related to unvested restricted stock awards granted was $14.6 million, which the Company expects to recognize over a weighted-average period of approximately 3.1 years.

Stock-Based Compensation Expense

Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows (in thousands):

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
Research and development	$	2,727	$	144	$	6,483	$	200
General and administrative		3,001		18,025		7,474		19,948
Total stock-based compensation	$	5,728	$	18,169	$	13,957	$	20,148

For the three months ended September 30, 2019 and 2018, $5.7 million and $18.2 million, respectively, of the stock-based compensation expense related to the issuance of anti-dilutive shares. For the nine months ended September 30, 2019 and 2018, $14.0 million and $20.1 million, respectively, of the stock-based compensation expense related to the vesting of anti-dilution shares.

As of September 30, 2019, total unrecognized compensation expense related to the ESPP was $1.7 million, which the Company expects to recognize over a weighted-average period of approximately 1.2 years.

Commitments and Contingencies

9 Months Ended

Sep. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Leases

The Company subleases certain office and laboratory space under a non-cancelable operating lease expiring in December 2024 for the initial leased space and December 2022 for expansion space leased pursuant to an amendment to the lease agreement entered into in August 2018. The sublease agreement included options to extend for the entire premises through October 2028. The options to extend must be exercised prior to the termination of the original lease agreement. The period covered by the options was not included in the non-cancellable lease term as it not was not determined to be reasonably certain to be executed. The lease agreement also includes a one-time termination option for the expansion space only whereby the Company can terminate the lease with advance written notice. The termination option was not determined to be reasonably certain to be executed. The lease is subject to charges for common area maintenance and other costs, and base rent is subject to an annual 3% increase each subsequent year. Costs determined to be variable and not based on an index or rate were not included in the measurement of the operating lease liabilities.

Monthly rent expense is recognized on a straight-line basis over the term of the lease. The operating lease is included in the balance sheet at the present value of the lease payments at a 7% discount rate using the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment as the leases do not provide an implicit rate. The weighted average remaining lease term was 4.5 years.

Lease costs were comprised of the following (in thousands):

	Three months ended September 30, 2019		Nine months ended September 30, 2019

Operating lease cost	$	753	$	2,258
Variable lease cost		374		1,126
Short-term lease cost		13		42
Total lease cost	$	1,140	$	3,426

Cash paid for amounts included in the measurement of operating lease liabilities for the three and nine months ended September 30, 2019 was $0.7 million and $2.2 million, respectively.

Gross future minimum annual rental commitments as of September 30, 2019, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2019 (remaining 3 months)	$	739
2020		3,038
2021		3,127
2022		3,220
2023		1,694
2024		1,745
Total undiscounted rent payments	$	13,563

Present value discount		(1,930	)
Present value	$	11,633
Current portion of operating lease liability (included as a component of accrued expenses)	$	2,291
Noncurrent operating lease liabilities		9,342
Total operating lease liability	$	11,633

Future minimum lease payments under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):

Years ending December 31,
2019	$	2,944
2020		3,035
2021		3,123
2022		3,216
2023		1,690
Thereafter		1,741
	$	15,749

For the three and nine months ended September 30, 2018 the Company recorded approximately $0.6 million and $1.5 million, respectively, in rent expense.

Litigation

The Company is not a party to any material legal proceedings and is not aware of any pending or threatened claims. From time to time, the Company may be subject to various legal proceedings and claims that arise in the ordinary course of its business activities.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Basis of Presentation

Use of Estimates

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements – Not Yet Adopted

Net Loss Per Share

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of September 30,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		10,158,710
Shares issuable upon conversion of Series B Convertible Preferred Stock		—		15,890,306
Shares issuable upon exercise of stock options		8,099,861		2,104,311
Non-vested shares under restricted stock grants		5,053,169		7,799,605

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2019

Accounting Policies [Abstract]

Schedule of Potentially Dilutive Securities not Included in Calculation of Diluted Net Loss Per Share

The table below provides potentially dilutive securities not included in the calculation of the diluted net loss per share because to do so would be anti-dilutive:

	As of September 30,
	2019		2018
Shares issuable upon conversion of Series Seed Convertible Preferred Stock		—		4,444,444
Shares issuable upon conversion of Series A Convertible Preferred Stock		—		10,158,710
Shares issuable upon conversion of Series B Convertible Preferred Stock		—		15,890,306
Shares issuable upon exercise of stock options		8,099,861		2,104,311
Non-vested shares under restricted stock grants		5,053,169		7,799,605

Balance Sheet Accounts and Supplemental Disclosures (Tables)

9 Months Ended

Sep. 30, 2019

Balance Sheets Accounts And Supplemental Disclosures [Abstract]

Schedule of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	Estimated Useful Life (in years)	September 30, 2019		December 31, 2018
Office equipment	3-7	$	1,097	$	918
Computer equipment	5		112		15
Software	3		78		50
Lab equipment	2-5		2,530		1,070
Leasehold improvements	6-7		2,170		1,243
Construction in process	N/A		68		194
Total property and equipment			6,055		3,490
Less: accumulated depreciation			922		297
Property and equipment, net		$	5,133	$	3,193

Schedule of Accrued Expenses

Accrued expenses consisted of the following (in thousands):

	As of
	September 30, 2019		December 31, 2018
Accrued compensation	$	6,365	$	4,102
Operating lease liabilities		2,291		—
Accrued professional service fees		2,059		2,697
Accrued other		728		769
Total accrued expenses	$	11,443	$	7,568

Fair Value Measurements and Available for Sale Investments (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Measurements And Available For Sale Investments [Abstract]

Assets Measured at Fair Value on Recurring Basis

The following table presents the hierarchy for assets measured at fair value on a recurring basis as of September 30, 2019 and December 31, 2018 (in thousands):

	Fair Value Measurements at End of Period Using:
				Quoted Market		Significant		Significant
				Prices for		Other Observable		Unobservable
		Total		Identical Assets		Inputs		Inputs
		Fair Value		(Level 1)		(Level 2)		(Level 3)
As of September 30, 2019
Money market funds		$	35,756	$	35,756	$	—	$	—
U.S. Treasury and agency securities			199,500		199,500		—		—
Commercial paper			30,985		—		30,985		—
Corporate debt securities			99,892		—		99,892		—
As of December 31, 2018
Money market funds		$	17,295	$	17,295	$	—	$	—
U.S. Treasury securities			123,439		123,439		—		—

Schedule of Available for Sale Investments by Security Type

			Gross		Gross
	Amortized		Unrealized		Unrealized			Total
	Cost		Gains		Losses			Fair Value
Marketable securities
U.S. Treasury and agency securities	$	185,844	$	145	$	(4	)	$	185,985
Commercial paper		29,887		—		—			29,887
Corporate debt securities		99,578		319		(5	)		99,892
Total marketable securities	$	315,309	$	464	$	(9	)	$	315,764

Schedule of Contractual Maturities of Available-for-sale Debt Securities

Contractual maturities of available-for-sale debt securities, as of September 30, 2019, were as follows (in thousands):

	Estimated
	Fair Value
Due within one year	$	290,813
One to two years		24,951
Total	$	315,764

Long-Term Debt (Tables)

9 Months Ended

Sep. 30, 2019

Debt Disclosure [Abstract]

Schedule of Long-Term Debt

Our long-term debt as of September 30, 2019 consisted of the following (in thousands):

	September 30, 2019
Term loan	$	30,000
Debt discount and issuance costs		(1,630	)
Long-term debt	$	28,370

Schedule of Future Minimum Principal Payments

The scheduled future minimum principal payments are as follows (in thousands)

	September 30, 2019
2019 (three-months)	$	—
2020		—
2021		5,833
2022		10,000
2023		10,000
2024		4,167
Total	$	30,000

Asset Acquisitions and Contingent Consideration (Tables)

9 Months Ended

Sep. 30, 2019

Business Combinations [Abstract]

Schedule of IPR&D Expense

The Company recorded the following IPR&D expense on the condensed consolidated statements of operations (in thousands):

	Three months ended September 30,				Nine months ended September 30,
	2019		2018		2019		2018
GB001	$	—	$	—	$	—	$	19,148
GB004		—		—		—		20,000
GB1275		—		7,500		1,000		7,500
Other Programs		—		761		1,000		3,011
Total in process research and development	$	—	$	8,261	$	2,000	$	49,659

Equity Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity during the nine months ended September 30, 2019:

	Shares Subject to Options Outstanding					Weighted- Average
				Weighted-		Remaining
				Average Exercise		Contractual Life		Aggregate
	Shares			Price		(Years)		Intrinsic Value
								(in thousands)
Outstanding as of December 31, 2018		5,107,329		$	7.51		9.7	$	16,343
Options granted		3,441,901		$	20.79
Option exercised		(173,712	)	$	3.04
Options forfeited/cancelled		(275,657	)	$	12.30
Outstanding as of September 30, 2019		8,099,861		$	13.09		9.2	$	44,015
Options vested and exercisable as of September 30, 2019		644,722		$	3.88		8.7	$	8,323

Summary of Restricted Stock Activity

The summary of the Company’s restricted stock activity is as follows:

	Number of			Weighted-
	Restricted			Average
	Stock Units			Grant Date
	Outstanding			Fair Value
Nonvested at December 31, 2018		7,482,032		$	4.01
Granted		—			—
Vested		(2,428,863	)	$	4.11
Forfeited		—			—
Nonvested at September 30, 2019		5,053,169		$	3.96

Stock-Based Compensation Expense Reported in Condensed Consolidated Statements of Operations

Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows (in thousands):

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
Research and development	$	2,727	$	144	$	6,483	$	200
General and administrative		3,001		18,025		7,474		19,948
Total stock-based compensation	$	5,728	$	18,169	$	13,957	$	20,148

Commitments and Contingencies (Tables)

9 Months Ended

Sep. 30, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Lease Costs

Lease costs were comprised of the following (in thousands):

	Three months ended September 30, 2019		Nine months ended September 30, 2019

Operating lease cost	$	753	$	2,258
Variable lease cost		374		1,126
Short-term lease cost		13		42
Total lease cost	$	1,140	$	3,426

Schedule of Gross Future Minimum Annual Rental Commitments

Gross future minimum annual rental commitments as of September 30, 2019, were as follows (in thousands):

	Undiscounted Rent Payments
Year ending December 31,
2019 (remaining 3 months)	$	739
2020		3,038
2021		3,127
2022		3,220
2023		1,694
2024		1,745
Total undiscounted rent payments	$	13,563

Present value discount		(1,930	)
Present value	$	11,633
Current portion of operating lease liability (included as a component of accrued expenses)	$	2,291
Noncurrent operating lease liabilities		9,342
Total operating lease liability	$	11,633

Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases

Future minimum lease payments under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):

Years ending December 31,
2019	$	2,944
2020		3,035
2021		3,123
2022		3,216
2023		1,690
Thereafter		1,741
	$	15,749

Description of the Business - Additional Information (Detail)				18 Months Ended
	May 02, 2019 USD ($) Tranche	Feb. 12, 2019 USD ($) $ / shares shares	Jan. 04, 2018 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) Tranche	Dec. 31, 2018 USD ($)
Description Of Business [Line Items]
Accumulated deficit					$ (279,472,000)	$ (153,863,000)
Funds raised through Series A and Series B Convertible Preferred Stock, convertible note financings and completed IPO				$ 601,300,000
Term Loan
Description Of Business [Line Items]
Maximum borrowing capacity	$ 150,000,000
Amount funded	$ 30,000,000
Number of additional tranches executed \| Tranche	3				0
Remaining borrowing capacity	$ 120,000,000
Term Loan \| Tranche One
Description Of Business [Line Items]
Remaining borrowing capacity	40,000,000
Term Loan \| Tranche Two
Description Of Business [Line Items]
Remaining borrowing capacity	30,000,000
Term Loan \| Tranche Three
Description Of Business [Line Items]
Remaining borrowing capacity	$ 50,000,000
AA Biopharma Inc.
Description Of Business [Line Items]
Cash received in connection with acquisition			$ 12,800,000
Initial Public Offering ("IPO")
Description Of Business [Line Items]
Sale of shares of common stock \| shares		19,837,500
Offering price, per share \| $ / shares		$ 16.00
Net proceeds, after deducting underwriting discounts, commissions, and offering expenses		$ 291,300,000
Initial Public Offering ("IPO") \| Conversion of Convertible Preferred Stock into Common Stock
Description Of Business [Line Items]
Common stock issued in conversion of convertible preferred stock \| shares		30,493,460

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities not Included in Calculation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Shares Issuable upon Conversion of Series Seed Convertible
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share		4,444,444
Shares Issuable upon Conversion of Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share		10,158,710
Shares Issuable upon Conversion of Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share		15,890,306
Shares Issuable upon Exercise of Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share	8,099,861	2,104,311
Non-vested Shares under Restricted Stock Grants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities not included from calculation of diluted net loss per share	5,053,169	7,799,605

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,055	$ 3,490
Less: accumulated depreciation	922	297
Property and equipment, net	5,133	3,193
Office Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 1,097	918
Office Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	3 years
Office Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 112	15
Property and equipment, estimated useful life	5 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 78	50
Property and equipment, estimated useful life	3 years
Lab Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 2,530	1,070
Lab Equipment \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	2 years
Lab Equipment \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	5 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 2,170	1,243
Leasehold Improvements \| Minimum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	6 years
Leasehold Improvements \| Maximum
Property Plant And Equipment [Line Items]
Property and equipment, estimated useful life	7 years
Construction in Process
Property Plant And Equipment [Line Items]
Total property and equipment	$ 68	$ 194

Balance Sheet Accounts and Supplemental Disclosures - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued compensation	$ 6,365	$ 4,102
Operating lease liabilities	2,291
Accrued professional service fees	2,059	2,697
Accrued other	728	769
Total accrued expenses	$ 11,443	$ 7,568

Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 35,756	$ 17,295
U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,500
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	30,985
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	99,892
U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis		123,439
Quoted Market Prices for Identical Assets (Level 1) \| Money Market Funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	35,756	17,295
Quoted Market Prices for Identical Assets (Level 1) \| U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	199,500
Quoted Market Prices for Identical Assets (Level 1) \| U.S. Treasury Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis		$ 123,439
Significant Other Observable Inputs (Level 2) \| Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	30,985
Significant Other Observable Inputs (Level 2) \| Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Assets Measured at fair value on recurring basis	$ 99,892

Fair Value Measurements and Available for Sale Investments - Additional Information (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value hierarchy level 1 to level 2	$ 0	$ 0
Fair value hierarchy level 2 to level 1	0	0
Cash Equivalents, at Carrying Value	14,600,000
Prepaid Expenses and Other Current Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Interest and securities receivable	$ 1,200,000	$ 600,000

Fair Value Measurements and Available for Sale Investments - Schedule of Available for sale Investments by Security Type (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	$ 315,309
Available-for-sale Investments, Gross Unrealized Gains	464
Available-for-sale Investments, Gross Unrealized Losses	(9)
Available-for-sale Investments, Total Fair Value	315,764
U S Treasury and Agency Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	185,844
Available-for-sale Investments, Gross Unrealized Gains	145
Available-for-sale Investments, Gross Unrealized Losses	(4)
Available-for-sale Investments, Total Fair Value	185,985
Commercial Paper
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	29,887
Available-for-sale Investments, Total Fair Value	29,887
Corporate Debt Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale Investments, Amortized Cost	99,578
Available-for-sale Investments, Gross Unrealized Gains	319
Available-for-sale Investments, Gross Unrealized Losses	(5)
Available-for-sale Investments, Total Fair Value	$ 99,892

Long-Term Debt - Additional Information (Details) - Term Loan		9 Months Ended
	May 02, 2019 USD ($) Tranche	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 150,000,000
Amount funded	$ 30,000,000
Number of additional tranches \| Tranche	3
Remaining borrowing capacity	$ 120,000,000
Interest only payment period	36 months
Maturity date	May 01, 2024
Percentage of exit fee on amount borrowed on final repayment	1.75%
Percentage of exit fee on amount borrowed on partial prepayment	1.75%
Debt covenant minimum unrestricted cash percentage	25.00%
Minimum indebtedness amount		$ 2,500,000
Prepayment Occurs through First Anniversary of Closing Date
Debt Instrument [Line Items]
Percentage of prepayment fee	3.00%
First Anniversary of Closing Date through Second Anniversary of Closing Date
Debt Instrument [Line Items]
Percentage of prepayment fee	2.00%
Second Anniversary of Closing Date and Prior to May 1, 2024
Debt Instrument [Line Items]
Percentage of prepayment fee	1.00%
LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	6.15%
Interest rate	2.00%
Debt instrument, description of variable rate basis	Each term loan under the Credit Facility bears interest at an annual rate equal to the sum of (i) one-month LIBOR (customarily defined, with a change to prime rate if LIBOR funding becomes unlawful or impractical) plus (ii) 6.15%, subject to a LIBOR floor of 2.00%.
MidCap Financial Trust
Debt Instrument [Line Items]
Basis spread on variable rate		3.00%
Tranche One
Debt Instrument [Line Items]
Remaining borrowing capacity	$ 40,000,000
Tranche Two
Debt Instrument [Line Items]
Remaining borrowing capacity	30,000,000
Tranche Three
Debt Instrument [Line Items]
Remaining borrowing capacity	$ 50,000,000

Long-Term Debt - Schedule of Long-term Debt (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Debt Disclosure [Abstract]
Term loan	$ 30,000
Debt discount and issuance costs	(1,630)
Long-term debt	$ 28,370

Long-Term Debt - Schedule of Future Minimum Principal Payments (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Debt Disclosure [Abstract]
2021	$ 5,833
2022	10,000
2023	10,000
2024	4,167
Total	$ 30,000

Convertible Notes Financing - Additional Information (Details) - Convertible Promissory Note - USD ($)	Jan. 04, 2018	Oct. 02, 2017
Debt Instrument [Line Items]
Amount of issued		$ 6,000,000
Interest rate		8.00%
Maturity date		Oct. 02, 2018
Amount to be raised by automatic conversion of the Note upon qualified equity financing		$ 40,000,000
Series A Convertible Preferred Stock
Debt Instrument [Line Items]
Number of shares issued for conversion of the Note	3,499,209

Asset Acquisitions and Contingent Consideration - Additional Information (Details) - USD ($)								1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 21, 2018	Jun. 30, 2018	Jan. 31, 2018	Jan. 04, 2018	Oct. 31, 2017	Oct. 02, 2017	May 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 24, 2018
Business Acquisition [Line Items]
In process research and development									$ 8,261,000	$ 2,000,000	$ 49,659,000
GB001
Business Acquisition [Line Items]
In process research and development											19,148,000
GB004
Business Acquisition [Line Items]
In process research and development											20,000,000
GB1275
Business Acquisition [Line Items]
In process research and development									$ 7,500,000	1,000,000	$ 7,500,000
Pulmokine, Inc. \| License Agreement \| GB002
Business Acquisition [Line Items]
Product license term							10 years
Upfront payment						$ 5,500,000
Milestones accrued										0
Pulmokine, Inc. \| License Agreement \| GB002 \| Maximum
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable							$ 63,000,000
Commercial milestone payments, payable							45,000,000
Sales milestone payments, payable							$ 190,000,000
AA Biopharma Inc. \| GB001
Business Acquisition [Line Items]
In process research and development				$ 19,300,000
AA Biopharma Inc. \| GB001 \| Common Stock
Business Acquisition [Line Items]
Business acquisition, aggregate number of shares issued					1,101,278
AA Biopharma Inc. \| GB001 \| Series Seed Convertible Preferred Stock
Business Acquisition [Line Items]
Business acquisition, aggregate number of shares issued					20,000,000
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004
Business Acquisition [Line Items]
Milestones accrued										0
Upfront payment, purchase consideration paid			$ 20,000,000
Aerpio Pharmaceuticals, Inc. \| License Agreement \| GB004 \| Maximum
Business Acquisition [Line Items]
Development and regulatory milestone payments, payable												$ 55,000,000
Commercial milestone payments, payable												85,000,000
Sales milestone payments, payable												$ 260,000,000
Adhaere Pharmaceuticals, Inc. \| GB1275
Business Acquisition [Line Items]
Milestones accrued										$ 0
In process research and development	$ 7,500,000
Upfront payment, purchase consideration paid		$ 7,500,000
Milestones payment								$ 1,000,000
Adhaere Pharmaceuticals, Inc. \| GB1275 \| Maximum
Business Acquisition [Line Items]
Regulatory, development and sales milestone payments payable		$ 62,000,000

Stockholders' Equity (Deficit) - Additional Information (Details) - USD ($)					1 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 06, 2018	May 21, 2018	Jan. 04, 2018	Dec. 03, 2015	Jan. 31, 2018	Nov. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Stockholders Equity Deficit [Line Items]
Common stock, par value									$ 0.0001	$ 0.0001
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs							$ 291,344,000
Common stock, voting right									one
Shares of common stock, repurchase									5,053,169	7,482,032
Series A Convertible Preferred Stock \| Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Shares subject to forfeiture						1,305,427
Founder
Stockholders Equity Deficit [Line Items]
Common stock issued shares	1,795,023	251,547
Percentage of fully diluted share capital			15.00%
Increased in equity capital amount			$ 300,000,000
Common stock issued price per share	$ 9.63	$ 2.61
Additional shares of restricted stock subject to vesting restrictions	1,795,023	251,547
Common Stock
Stockholders Equity Deficit [Line Items]
Conversion of convertible preferred stock into common stock, shares							30,493,460		30,493,460
Common stock issued shares							19,837,500
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs							$ 2,000
Unrestricted shares of common stock were subject to service vesting conditions								4,580,444
Common Stock \| Stock Restriction Agreements
Stockholders Equity Deficit [Line Items]
Unrestricted shares of common stock were subject to service vesting conditions					4,580,444
Founder Shares
Stockholders Equity Deficit [Line Items]
Common stock issued shares				9,160,888
Common stock, par value				$ 0.0001
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs				$ 4,100
Common Stock vesting percentage			50.00%
Common stock vesting period			5 years

Equity Incentive Plans - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Feb. 06, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares outstanding awarded		8,099,861		8,099,861		5,107,329
Stock-based compensation expense related to vesting of anti-dilution shares		$ 5,728	$ 18,169	$ 13,957	$ 20,148
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards granted				0
Weighted-average period of cost expects to recognize				3 years 1 month 6 days
Unrecognized compensation costs		14,600		$ 14,600
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield				0.00%
Weighted-average grant date fair value per share				$ 20.79
Unrecognized compensation costs		57,800		$ 57,800
Weighted-average period of cost expects to recognize				3 years
Stock-Based Compensation Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average period of cost expects to recognize				1 year 2 months 12 days
Stock-based compensation expense related to vesting of anti-dilution shares		$ 5,700	$ 18,200	$ 14,000	$ 20,100
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share reserved for future issuance	5,750,000
Term of awards	10 years
Percentage of amount increase in outstanding shares	5.00%
Common stock available for issuance		2,486,308		2,486,308
Shares outstanding awarded		3,263,692		3,263,692
2019 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of share reserved for future issuance	700,000	700,000		700,000
Term of awards	10 years
Percentage of amount increase in outstanding shares	1.00%
2019 Employee Stock Purchase Plan \| Stock-Based Compensation Expense
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation costs		$ 1,700		$ 1,700
2019 Employee Stock Purchase Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase of common stock through payroll deductions, percentage	20.00%
2017 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares outstanding awarded		4,819,318		4,819,318
2017 Equity Incentive Plan \| Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock awards granted				635,155

Equity Incentive Plans - Summary of Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Subject to Options Outstanding, as of December 31, 2018 \| shares	5,107,329
Shares Subject to Options Outstanding, Options granted \| shares	3,441,901
Shares Subject to Options Outstanding, Option exercised \| shares	(173,712)
Shares Subject to Options Outstanding, Options forfeited/cancelled \| shares	(275,657)
Shares Subject to Options Outstanding, Outstanding as of September 30, 2019 \| shares	8,099,861	5,107,329
Shares Subject to Options Outstanding, Options vested and exercisable as of September 30, 2019 \| shares	644,722
Weighted-Average Exercise Price, Outstanding as of December 31, 2018 \| $ / shares	$ 7.51
Weighted-Average Exercise Price, Options granted \| $ / shares	20.79
Weighted-Average Exercise Price, Option exercised \| $ / shares	3.04
Weighted-Average Exercise Price, Options forfeited/cancelled \| $ / shares	12.30
Weighted-Average Exercise Price, Outstanding as of September 30, 2019 \| $ / shares	13.09	$ 7.51
Weighted-Average Exercise Price, Options vested and exercisable as of September 30, 2019 \| $ / shares	$ 3.88
Weighted-Average Remaining Contractual Life (Years), Outstanding as of December 31, 2018	9 years 2 months 12 days	9 years 8 months 12 days
Weighted-Average Remaining Contractual Life (Years), Options vested and exercisable as of September 30, 2019	8 years 8 months 12 days
Aggregate Intrinsic Value, Outstanding as of December 31, 2018 \| $	$ 44,015	$ 16,343
Aggregate Intrinsic Value, Options vested and exercisable as of September 30, 2019 \| $	$ 8,323

Equity Incentive Plans - Summary of Restricted Stock Activity (Details) - Restricted Stock	9 Months Ended
	Sep. 30, 2019 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Restricted Stock Units Outstanding, Nonvested at December 31, 2018 \| shares	7,482,032
Number of Restricted Stock Units Outstanding, Granted \| shares	0
Number of Restricted Stock Units Outstanding, Vested \| shares	(2,428,863)
Number of Restricted Stock Units Outstanding, Forfeited \| shares	0
Number of Restricted Stock Units Outstanding, Nonvested at September 30, 2019 \| shares	5,053,169
Weighted-Average Grant Date Fair Value, Nonvested at December 31, 2018 \| $ / shares	$ 4.01
Weighted-Average Grant Date Fair Value, Granted \| $ / shares	0
Weighted-Average Grant Date Fair Value, Vested \| $ / shares	4.11
Weighted-Average Grant Date Fair Value, Forfeited \| $ / shares	0
Weighted-Average Grant Date Fair Value, Nonvested at September 30, 2019 \| $ / shares	$ 3.96

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Non-cancelable operating sublease, expiration date for initial leased space			2024-12
Non-cancelable operating sublease, expiration date for expansion space leased			2022-12
Lessee, Operating Sublease, Existence of Option to Extend [true false]			true
Lessee, operating sublease, option to extend			options to extend for the entire premises through October 2028
Operating sublease, annual increase percentage for base rent			3.00%
Lessee, operating lease, discount rate	7.00%		7.00%
Weighted average remining lease term	4 years 6 months		4 years 6 months
Cash paid for operating lease liabilities	$ 0.7		$ 2.2
Rent expense		$ 0.6		$ 1.5

Commitments and Contingencies - Schedule of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Leases [Abstract]
Operating lease cost	$ 753	$ 2,258
Variable lease cost	374	1,126
Short-term lease cost	13	42
Total lease cost	$ 1,140	$ 3,426

Commitments and Contingencies - Schedule of Gross Future Minimum Annual Rental Commitments (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Leases [Abstract]
2019 (remaining 3 months)	$ 739
2020	3,038
2021	3,127
2022	3,220
2023	1,694
2024	1,745
Total undiscounted rent payments	13,563
Present value discount	(1,930)
Present value	11,633
Current portion of operating lease liability (included as a component of accrued expenses)	2,291
Noncurrent operating lease liabilities	9,342
Total operating lease liability	$ 11,633

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Leases [Abstract]
2019	$ 2,944
2020	3,035
2021	3,123
2022	3,216
2023	1,690
Thereafter	1,741
Total	$ 15,749

Condensed Consolidated Statements of Cash Flows (Unaudited) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Statement Of Cash Flows [Abstract]
Debt issuance costs	$ 1,778

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 10,860
Operating lease liabilities	$ 11,633
Accounting Standards Update 2016-02
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Operating lease right-of-use assets		$ 12,500
Operating lease liabilities		$ 13,200

Fair Value Measurements and Available for Sale Investments - Schedule of Contractual Maturities of Available-for-sale Debt Securities (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Fair Value Disclosures [Abstract]
Due within one year	$ 290,813
One to two years	24,951
Total	$ 315,764