CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total		Common Stock	Additional Paid-In Capital	Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2019			66,886,000
Beginning balance at Dec. 31, 2019	$ 330,719		$ 67	$ 526,771	$ 25	$ (196,144)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)			984,000
Issuance of common stock upon exercise of stock options	3,591		$ 1	3,590
Vesting of common stock from early exercises (in shares)			21,000
Vesting of common stock from early exercises	162			162
Stock-based compensation expense	3,695			3,695
Comprehensive income (loss)	(80)				(80)
Net loss	(19,115)					(19,115)
Ending balance (in shares) at Mar. 31, 2020			67,891,000
Ending balance at Mar. 31, 2020	318,972		$ 68	534,218	(55)	(215,259)
Beginning balance (in shares) at Dec. 31, 2019			66,886,000
Beginning balance at Dec. 31, 2019	330,719		$ 67	526,771	25	(196,144)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(74,496)
Ending balance (in shares) at Sep. 30, 2020			68,914,000
Ending balance at Sep. 30, 2020	276,703		$ 69	547,259	15	(270,640)
Beginning balance (in shares) at Mar. 31, 2020			67,891,000
Beginning balance at Mar. 31, 2020	318,972		$ 68	534,218	(55)	(215,259)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)			680,000
Issuance of common stock upon exercise of stock options	2,366		$ 1	2,365
Issuance of common stock under employee stock purchase plan (in shares)			109,000
Issuance of common stock under employee stock purchase plan	1,285			1,285
Issuance of common stock under 401(k) Plan (in shares)			6,000
Issuance of common stock to participants in 401(k) plan	119			119
Vesting of common stock from early exercises (in shares)			16,000
Vesting of common stock from early exercises	123			123
Stock-based compensation expense	3,723			3,723
Comprehensive income (loss)	192				192
Net loss	(25,616)					(25,616)
Ending balance (in shares) at Jun. 30, 2020			68,702,000
Ending balance at Jun. 30, 2020	301,164		$ 69	541,833	137	(240,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)			195,000
Issuance of common stock upon exercise of stock options	1,257			1,257
Vesting of common stock from early exercises (in shares)			17,000
Vesting of common stock from early exercises	120			120
Stock-based compensation expense	4,049			4,049
Comprehensive income (loss)	(122)				(122)
Net loss	(29,765)					(29,765)
Ending balance (in shares) at Sep. 30, 2020			68,914,000
Ending balance at Sep. 30, 2020	276,703		$ 69	547,259	15	(270,640)
Beginning balance (in shares) at Dec. 31, 2020			70,583,000
Beginning balance at Dec. 31, 2020	280,043	[1]	$ 71	578,599	4	(298,631)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under offering, net of issuance cost (in shares)			5,324,000
Issuance of common stock under offering, net of issuance costs	134,570		$ 5	134,565
Issuance of common stock upon exercise of stock options (in shares)			1,001,000
Issuance of common stock upon exercise of stock options	5,907		$ 1	5,906
Vesting of common stock from early exercises (in shares)			5,000
Vesting of common stock from early exercises	41			41
Stock-based compensation expense	6,582			6,582
Comprehensive income (loss)	(22)				(22)
Net loss	(27,544)					(27,544)
Ending balance (in shares) at Mar. 31, 2021			76,913,000
Ending balance at Mar. 31, 2021	399,577		$ 77	725,693	(18)	(326,175)
Beginning balance (in shares) at Dec. 31, 2020			70,583,000
Beginning balance at Dec. 31, 2020	$ 280,043	[1]	$ 71	578,599	4	(298,631)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	1,627,867
Net loss	$ (93,101)
Ending balance (in shares) at Sep. 30, 2021			77,662,000
Ending balance at Sep. 30, 2021	354,039		$ 78	745,704	(11)	(391,732)
Beginning balance (in shares) at Mar. 31, 2021			76,913,000
Beginning balance at Mar. 31, 2021	399,577		$ 77	725,693	(18)	(326,175)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock under offering, net of issuance costs	10			10
Issuance of common stock upon exercise of stock options (in shares)			280,000
Issuance of common stock upon exercise of stock options	1,905			1,905
Issuance of common stock under employee stock purchase plan (in shares)			110,000
Issuance of common stock under employee stock purchase plan	1,409			1,409
Issuance of common stock under 401(k) Plan (in shares)			4,000
Issuance of common stock to participants in 401(k) plan	125			125
Vesting of common stock from early exercises	2			2
Stock-based compensation expense	6,716			6,716
Comprehensive income (loss)	23				23
Net loss	(36,692)					(36,692)
Ending balance (in shares) at Jun. 30, 2021			77,307,000
Ending balance at Jun. 30, 2021	373,075		$ 77	735,860	5	(362,867)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)			348,000
Issuance of common stock upon exercise of stock options	3,282		$ 1	3,281
Issuance of common stock shares, under Open Market Agreement, net of issuance cost (in shares)			7,000
Issuance of common stock under Open Market Agreement, net of issuance costs	196			196
Vesting of common stock from early exercises	2			2
Stock-based compensation expense	6,365			6,365
Comprehensive income (loss)	(16)				(16)
Net loss	(28,865)					(28,865)
Ending balance (in shares) at Sep. 30, 2021			77,662,000
Ending balance at Sep. 30, 2021	$ 354,039		$ 78	$ 745,704	$ (11)	$ (391,732)

[1] The condensed consolidated balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.