Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series B-2 Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2016		58,600,315
Convertible preferred stock, beginning balance at Dec. 31, 2016		$ 348,673
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net of issuance costs (in shares)			1,764,705
Issuance of convertible preferred stock, net of issuance costs			$ 29,927
Conversion of Series A and B convertible preferred stock into common stock (in shares)		(60,365,020)
Conversion of Series A and B convertible preferred stock into common stock		$ (378,600)
Convertible preferred stock, ending balance (in shares) at Dec. 31, 2017		0
Convertible preferred stock, ending balance at Dec. 31, 2017		$ 0
Common stock, shares outstanding beginning balance (in shares) at Dec. 31, 2016				8,597,316
Beginning balance at Dec. 31, 2016	$ (94,154)			$ 344	$ 9,387	$ (373)	$ (103,512)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Convertible preferred stock converted into shares of common stock (in shares)				60,365,020
Convertible preferred stock converted into shares of common stock	378,600			$ 604	377,996
Issuance of common stock upon initial public offering, net of issuance costs (in shares)				15,972,221
Issuance of common stock upon initial public offering, net of issuance costs	264,261			$ 160	264,101
Issuance of common stock upon exercise of stock options (in shares)				648,317
Issuance of common stock upon exercise of stock options	733			$ 25	708
Issuance of reversed common stock as consideration in asset acquisition (in shares)				81,164
Issuance of reserved common stock as consideration in asset acquisition	0			$ 1	(1)
Vesting of early exercised common stock (in shares)				187,500
Vesting of early exercised common stock	127			$ 6	121
Vesting of restricted stock awards (in shares)				1,628,824
Vesting of restricted stock awards	0			$ 61	(61)
Stock-based compensation	4,409				4,409
Net loss	(88,185)						(88,185)
Other comprehensive income (loss)	5					5
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2017				87,480,362
Ending balance at Dec. 31, 2017	465,796			$ 1,201	656,660	(368)	(191,697)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Takeda Collaboration Agreement (in shares)				4,214,559
Issuance of common stock in connection with the Takeda Collaboration Agreement	94,406			$ 42	94,364
Issuances under equity incentive plans (in shares)				792,939
Issuances under equity incentive plans	3,999			$ 8	3,991
Vesting of early exercised common stock (in shares)				234,372
Vesting of early exercised common stock	374			$ 3	371
Vesting of restricted stock awards (in shares)				1,940,203
Vesting of restricted stock awards	0			$ 19	(19)
Stock-based compensation	18,791				18,791
Net loss	(36,240)						(36,240)
Other comprehensive income (loss)	$ (281)					(281)
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2018	94,662,435			94,662,435
Ending balance at Dec. 31, 2018	$ 546,845			$ 1,273	774,158	(649)	(227,937)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	786,638
Issuances under equity incentive plans (in shares)				973,012
Issuances under equity incentive plans	$ 6,190			$ 9	6,181
Vesting of early exercised common stock (in shares)				135,424
Vesting of early exercised common stock	92			$ 2	90
Vesting of restricted stock awards (in shares)				419,064
Vesting of restricted stock awards	0			$ 4	(4)
Stock-based compensation	38,378				38,378
Net loss	(197,614)						(197,614)
Other comprehensive income (loss)	$ 999					999
Common stock, shares outstanding ending balance (in shares) at Dec. 31, 2019	96,189,935			96,189,935
Ending balance at Dec. 31, 2019	$ 394,890			$ 1,288	$ 818,803	$ 350	$ (425,551)