Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		87,480,362
Beginning 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 (2018) restricted stock awards and units (2019) and restricted stock units (2020) (in shares)		1,940,203
Vesting of restricted stock awards (2018) restricted stock awards and units (2019) and restricted stock units (2020)	0	$ 19	(19)
Stock-based compensation	18,791		18,791
Net Income (loss)	(36,240)				(36,240)
Other comprehensive income (loss)	(281)			(281)
Ending balance (in shares) at Dec. 31, 2018		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]
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 (2018) restricted stock awards and units (2019) and restricted stock units (2020) (in shares)		419,064
Vesting of restricted stock awards (2018) restricted stock awards and units (2019) and restricted stock units (2020)	0	$ 4	(4)
Stock-based compensation	38,378		38,378
Net Income (loss)	(197,614)				(197,614)
Other comprehensive income (loss)	$ 999			999
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in follow-on offering, net of issuance costs of 632 (in shares)		9,000,000
Issuance of common stock in follow-on offering, net of issuance costs of $632	193,948	$ 90	193,858
Issuance of common stock in connection with the Biogen Stock Purchase Agreement (in shares)		13,310,243
Issuance of common stock in connection with the Biogen Stock Purchase Agreement	420,146	$ 133	420,013
Issuances under equity incentive plans (in shares)		1,722,058
Issuances under equity incentive plans	20,655	$ 17	20,638
Vesting of restricted stock awards (2018) restricted stock awards and units (2019) and restricted stock units (2020) (in shares)		309,097
Vesting of restricted stock awards (2018) restricted stock awards and units (2019) and restricted stock units (2020)	0	$ 3	(3)
Stock-based compensation	50,351		50,351
Net Income (loss)	71,136				71,136
Other comprehensive income (loss)	$ (595)			(595)
Ending balance (in shares) at Dec. 31, 2020	120,531,333	120,531,333
Ending balance at Dec. 31, 2020	$ 1,150,531	$ 1,531	$ 1,503,660	$ (245)	$ (354,415)