Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Purchase Agreement Gilead	Cumulative Effect, Period of Adoption, Adjustment	Convertible Preferred Stock	Common Stock	Common Stock Purchase Agreement Gilead	Additional Paid-In Capital	Additional Paid-In Capital Purchase Agreement Gilead	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2017	$ (72,328)						$ 948		$ (73,234)		$ (42)
Balance, shares at Dec. 31, 2017				30,459,574
Balance at Dec. 31, 2017				$ 226,196
Balance, shares at Dec. 31, 2017					3,278,129
Conversion of preferred stock to common stock	226,198				$ 3		226,195
Conversion of preferred stock to common stock, shares				(30,459,574)
Conversion of preferred stock to common stock				$ (226,196)
Conversion of preferred stock to common stock, shares					30,459,574
Issuance of common stock	124,735				$ 1		124,734
Issuance of common stock, shares					9,200,000
Issuance of common stock upon exercise of stock options	95						95
Issuance of common stock upon exercise of stock options, shares					67,349
Vesting of early exercised stock options and restricted stock	1,276						1,276
Vesting of early exercised stock options and restricted stock, shares					528,374
Issuance of common stock under Employee Stock Purchase Plan	751						751
Issuance of common stock under Employee Stock Purchase Plan, shares					77,397
Stock-based compensation	3,874						3,874
Other comprehensive income (loss)	(65)										(65)
Net loss	(49,594)								(49,594)
Balance at Dec. 31, 2018	234,942				$ 4		357,873		(122,828)		(107)
Balance, shares at Dec. 31, 2018					43,610,823
Issuance of common stock upon exercise of stock options	188						188
Issuance of common stock upon exercise of stock options, shares					34,780
Vesting of early exercised stock options and restricted stock	1,029						1,029
Vesting of early exercised stock options and restricted stock, shares					417,883
Issuance of common stock under Employee Stock Purchase Plan	1,029						1,029
Issuance of common stock under Employee Stock Purchase Plan, shares					148,709
Stock-based compensation	8,981						8,981
Other comprehensive income (loss)	171										171
Net loss	(84,710)								(84,710)
Balance at Dec. 31, 2019	$ 163,842				$ 4		369,100		(205,326)		64
Balance (ASC 606) at Dec. 31, 2019			$ 2,212							$ 2,212
Balance, shares at Dec. 31, 2019				0
Balance, shares at Dec. 31, 2019	45,925,004				44,212,195
Issuance of common stock	$ 326,246	$ 107,468			$ 2		326,244	$ 107,468
Issuance of common stock, shares					12,650,000	5,963,029
Issuance of common stock upon exercise of stock options, shares	392,523
Issuance of common stock upon exercise of stock options and vesting of restricted stock	$ 3,366						3,366
Issuance of common stock upon exercise of stock options and vesting of restricted stock, shares					405,752
Vesting of early exercised stock options and restricted stock	848						848
Vesting of early exercised stock options and restricted stock, shares					258,824
Issuance of common stock under Employee Stock Purchase Plan	1,587						1,587
Issuance of common stock under Employee Stock Purchase Plan, shares					202,101
Stock-based compensation	21,825						21,825
Other comprehensive income (loss)	(20)										(20)
Net loss	(122,858)								(122,858)
Balance at Dec. 31, 2020	$ 502,304				$ 6		$ 830,438		$ (328,184)		$ 44
Balance, shares at Dec. 31, 2020				0
Balance, shares at Dec. 31, 2020	65,114,685				63,691,901