CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive income	Accumulated deficit	Total
Balance at Dec. 31, 2019	$ 8	$ 531,937	$ 14	$ (524,785)	$ 7,174
Balance (in shares) at Dec. 31, 2019	80,117,531
Increase (Decrease) in Stockholders' Equity
Net (loss) income				(37,990)	(37,990)
Unrealized (loss) gain on available-for-sale securities			(5)		(5)
Conversion of Notes into common stock	$ 3	57,411			57,414
Conversion of Notes into common stock (in shares)	34,796,350
Issuance of common stock under Employee Stock Purchase Plan		259			259
Issuance of common stock under Employee Stock Purchase Plan (in shares)	227,141
Issuance of common stock resulting from vesting of restricted stock units		(51)			(51)
Issuance of common stock resulting from vesting of restricted stock units (in shares)	58,166
Issuance of common stock resulting from exercise of stock options		983			983
Issuance of common stock resulting from exercise of stock options (in shares)	645,628
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs	$ 5	93,824			93,829
Issuance of common stock resulting from private investment in public equity offering, net of issuance costs (in shares)	46,511,628
Stock-based compensation expense		1,370			1,370
Balance at Mar. 31, 2020	$ 16	685,733	9	(562,775)	122,983
Balance (in shares) at Mar. 31, 2020	162,356,444
Balance at Dec. 31, 2019	$ 8	531,937	14	(524,785)	7,174
Balance (in shares) at Dec. 31, 2019	80,117,531
Increase (Decrease) in Stockholders' Equity
Net (loss) income					(47,862)
Unrealized (loss) gain on available-for-sale securities					(14)
Balance at Sep. 30, 2020	$ 17	702,403		(572,647)	129,773
Balance (in shares) at Sep. 30, 2020	169,540,447
Balance at Mar. 31, 2020	$ 16	685,733	9	(562,775)	122,983
Balance (in shares) at Mar. 31, 2020	162,356,444
Increase (Decrease) in Stockholders' Equity
Net (loss) income				(23,010)	(23,010)
Unrealized (loss) gain on available-for-sale securities			(9)		(9)
Issuance of common stock resulting from vesting of restricted stock units		(31)			(31)
Issuance of common stock resulting from vesting of restricted stock units (in shares)	32,650
Issuance of common stock resulting from at-the-market transactions, net of issuance costs	$ 1	12,229			12,230
Issuance of common stock resulting from at-the-market transactions, net of issuance costs (in shares)	6,769,559
Issuance of common stock resulting from exercise of stock options		551			551
Issuance of common stock resulting from exercise of stock options (in shares)	179,266
Stock-based compensation expense		1,659			1,659
Balance at Jun. 30, 2020	$ 17	700,141		(585,785)	114,373
Balance (in shares) at Jun. 30, 2020	169,337,919
Increase (Decrease) in Stockholders' Equity
Net (loss) income				13,138	13,138
Issuance of common stock under Employee Stock Purchase Plan		148			148
Issuance of common stock under Employee Stock Purchase Plan (in shares)	131,052
Issuance of common stock resulting from vesting of restricted stock units		(42)			(42)
Issuance of common stock resulting from vesting of restricted stock units (in shares)	71,476
Stock-based compensation expense		2,156			2,156
Balance at Sep. 30, 2020	$ 17	702,403		(572,647)	129,773
Balance (in shares) at Sep. 30, 2020	169,540,447
Balance at Dec. 31, 2020	$ 17	707,715	53	(592,511)	$ 115,274
Balance (in shares) at Dec. 31, 2020	170,456,179				170,456,000
Increase (Decrease) in Stockholders' Equity
Net (loss) income				(15,031)	$ (15,031)
Unrealized (loss) gain on available-for-sale securities			(19)		(19)
Issuance of common stock under Employee Stock Purchase Plan		76			76
Issuance of common stock under Employee Stock Purchase Plan (in shares)	53,372
Issuance of common stock resulting from vesting of restricted stock units		(52)			(52)
Issuance of common stock resulting from vesting of restricted stock units (in shares)	1,047,271
Issuance of common stock resulting from exercise of stock options		381			381
Issuance of common stock resulting from exercise of stock options (in shares)	173,890
Stock-based compensation expense		1,980			1,980
Balance at Mar. 31, 2021	$ 17	710,100	34	(607,542)	102,609
Balance (in shares) at Mar. 31, 2021	171,730,712
Balance at Dec. 31, 2020	$ 17	707,715	53	(592,511)	$ 115,274
Balance (in shares) at Dec. 31, 2020	170,456,179				170,456,000
Increase (Decrease) in Stockholders' Equity
Net (loss) income					$ (54,719)
Unrealized (loss) gain on available-for-sale securities					(26)
Balance at Sep. 30, 2021	$ 18	742,840	27	(647,230)	$ 95,655
Balance (in shares) at Sep. 30, 2021	182,155,258				182,155,000
Balance at Mar. 31, 2021	$ 17	710,100	34	(607,542)	$ 102,609
Balance (in shares) at Mar. 31, 2021	171,730,712
Increase (Decrease) in Stockholders' Equity
Net (loss) income				(16,902)	(16,902)
Unrealized (loss) gain on available-for-sale securities			5		5
Issuance of common stock resulting from vesting of restricted stock units		(38)			(38)
Issuance of common stock resulting from vesting of restricted stock units (in shares)	15,896
Issuance of common stock resulting from exercise of stock options		361			361
Issuance of common stock resulting from exercise of stock options (in shares)	330,758
Stock-based compensation expense		2,170			2,170
Balance at Jun. 30, 2021	$ 17	712,593	39	(624,444)	88,205
Balance (in shares) at Jun. 30, 2021	172,077,366
Increase (Decrease) in Stockholders' Equity
Net (loss) income				(22,786)	(22,786)
Unrealized (loss) gain on available-for-sale securities			(12)		(12)
Conversion of Notes into common stock	$ 1	27,999			28,000
Conversion of Notes into common stock (in shares)	8,615,384
Issuance of common stock under Employee Stock Purchase Plan		106			106
Issuance of common stock under Employee Stock Purchase Plan (in shares)	56,688
Issuance of common stock resulting from vesting of restricted stock units (in shares)	1,327,445
Issuance of common stock resulting from exercise of stock options		155			155
Issuance of common stock resulting from exercise of stock options (in shares)	78,375
Stock-based compensation expense		1,987			1,987
Balance at Sep. 30, 2021	$ 18	$ 742,840	$ 27	$ (647,230)	$ 95,655
Balance (in shares) at Sep. 30, 2021	182,155,258				182,155,000