Condensed 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, 2019		96,189,935
Beginning 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 upon public offering, net of issuance costs of $632 (in shares)		9,000,000
Issuance of common stock upon public 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)		1,001,905
Issuances under equity incentive plans	11,133	$ 10	11,123
Vesting of restricted stock units (in shares)		293,133
Vesting of restricted stock units	0	$ 3	(3)
Stock-based compensation	36,521		36,521
Net loss	(173,749)				(173,749)
Other comprehensive loss	(339)			(339)
Ending balance (in shares) at Sep. 30, 2020		119,795,216
Ending balance at Sep. 30, 2020	882,550	$ 1,524	1,480,315	11	(599,300)
Beginning balance (in shares) at Jun. 30, 2020		105,897,872
Beginning balance at Jun. 30, 2020	502,167	$ 1,385	1,041,303	551	(541,072)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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)		412,389
Issuances under equity incentive plans	5,753	$ 4	5,749
Vesting of restricted stock units (in shares)		174,712
Vesting of restricted stock units	0	$ 2	(2)
Stock-based compensation	13,252		13,252
Net loss	(58,228)				(58,228)
Other comprehensive loss	(540)			(540)
Ending balance (in shares) at Sep. 30, 2020		119,795,216
Ending balance at Sep. 30, 2020	$ 882,550	$ 1,524	1,480,315	11	(599,300)
Beginning balance (in shares) at Dec. 31, 2020	120,531,333	120,531,333
Beginning balance at Dec. 31, 2020	$ 1,150,531	$ 1,531	1,503,660	(245)	(354,415)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		880,560
Issuances under equity incentive plans	14,180	$ 8	14,172
Vesting of restricted stock units (in shares)		599,258
Vesting of restricted stock units	0	$ 6	(6)
Stock-based compensation	63,162		63,162
Net loss	(215,320)				(215,320)
Other comprehensive loss	$ (276)			(276)
Ending balance (in shares) at Sep. 30, 2021	122,011,151	122,011,151
Ending balance at Sep. 30, 2021	$ 1,012,277	$ 1,545	1,580,988	(521)	(569,735)
Beginning balance (in shares) at Jun. 30, 2021		121,531,952
Beginning balance at Jun. 30, 2021	1,072,705	$ 1,541	1,556,679	(368)	(485,147)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuances under equity incentive plans (in shares)		228,048
Issuances under equity incentive plans	3,315	$ 2	3,313
Vesting of restricted stock units (in shares)		251,151
Vesting of restricted stock units	0	$ 2	(2)
Stock-based compensation	20,998		20,998
Net loss	(84,588)				(84,588)
Other comprehensive loss	$ (153)			(153)
Ending balance (in shares) at Sep. 30, 2021	122,011,151	122,011,151
Ending balance at Sep. 30, 2021	$ 1,012,277	$ 1,545	$ 1,580,988	$ (521)	$ (569,735)