Stockholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended						9 Months Ended
	Jul. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Common Stock
Balance	$ 182,282	$ 182,282	$ 194,166	$ 209,357	$ 238,444	$ 83,703	$ 94,026	$ 209,357	$ 94,026
Balance (in shares)				39,603,771				39,603,771
Shares issued under stock option and employee stock purchase plans		2,004		$ 74	194		24
Shares issued under stock option and employee stock purchase plans			(25)
Shares issued in connection with at the market agreement					4,297	23,692	1,614
Shares issued in underwritten offering, net		269,893				141,361
Stock-based compensation		3,029	1,509	1,275	1,250	722	686
Unrealized (loss) gain on marketable securities		(44)	5	(2)	14	(3)	(22)	$ (41)	(11)
Net loss		(20,452)	(13,373)	(16,538)	(14,224)	(11,031)	(12,625)	(50,363)	(37,880)
Balance		$ 436,712	182,282	194,166	229,975	238,444	83,703	$ 436,712	229,975
Balance (in shares)		46,663,340						46,663,340
Aggregate gross offering price available		$ 50,000						$ 50,000
Net proceeds from sale of common stock								269,893	170,964
Underwritten Public Offering
Common Stock
Common stock issued (in shares)	6,845,238
Net proceeds from sale of common stock	$ 269,900
Common Stock
Common Stock
Balance	$ 40	$ 40	$ 40	$ 40	$ 39	$ 18	$ 17	$ 40	$ 17
Balance (in shares)	39,616,696	39,616,696	39,614,638	39,603,771	39,093,868	17,730,802	16,972,077	39,603,771	16,972,077
Shares issued under stock option and employee stock purchase plans (in shares)		201,406		10,867	68,204		12,573
Shares issued under stock option and employee stock purchase plans (in shares)			2,058
Shares issued in connection with at the market agreement					$ 1	$ 6	$ 1
Shares issued in connection with at the market agreement (in shares)					400,400	5,978,452	746,152
Shares issued in underwritten offering, net		$ 7				$ 15
Shares issued in underwritten offering, net (in shares)		6,845,238				15,384,614
Balance		$ 47	$ 40	$ 40	$ 40	$ 39	$ 18	$ 47	$ 40
Balance (in shares)		46,663,340	39,616,696	39,614,638	39,562,472	39,093,868	17,730,802	46,663,340	39,562,472
Additional Paid-In Capital
Common Stock
Balance	$ 1,282,657	$ 1,282,657	$ 1,281,173	$ 1,279,824	$ 1,272,783	$ 1,107,029	$ 1,104,706	$ 1,279,824	$ 1,104,706
Shares issued under stock option and employee stock purchase plans		2,004		74	194		24
Shares issued under stock option and employee stock purchase plans			(25)
Shares issued in connection with at the market agreement					4,296	23,686	1,613
Shares issued in underwritten offering, net		269,886				141,346
Stock-based compensation		3,029	1,509	1,275	1,250	722	686
Balance		1,557,576	1,282,657	1,281,173	1,278,523	1,272,783	1,107,029	1,557,576	1,278,523
Accumulated Other Comprehensive Income
Common Stock
Balance	2,592	2,592	2,587	2,589	2,594	2,597	2,619	2,589	2,619
Unrealized (loss) gain on marketable securities		(44)	5	(2)	14	(3)	(22)
Balance		2,548	2,592	2,587	2,608	2,594	2,597	2,548	2,608
Accumulated Deficit
Common Stock
Balance	$ (1,103,007)	(1,103,007)	(1,089,634)	(1,073,096)	(1,036,972)	(1,025,941)	(1,013,316)	(1,073,096)	(1,013,316)
Net loss		(20,452)	(13,373)	(16,538)	(14,224)	(11,031)	(12,625)
Balance		$ (1,123,459)	$ (1,103,007)	$ (1,089,634)	$ (1,051,196)	$ (1,036,972)	$ (1,025,941)	$ (1,123,459)	$ (1,051,196)