STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Accumulated Deficit
Balance at Dec. 31, 2019	$ 95,010	$ 182	$ 778,953	$ (684,125)
Balance (in shares) at Dec. 31, 2019		181,803,320
Stock-based compensation	5,393		5,393
Exercise of employee stock options	43		43
Exercise of employee stock options (in shares)		22,916
Cancelled restricted common stock (in shares)		(141,230)
Issuance of restricted common stock		$ 1	(1)
Issuance of restricted common stock (in shares)		555,900
Issuance of common stock in connection with a public offering, net	88,661	$ 29	88,632
Issuance of common stock in connection with a public offering, net (in shares)		29,110,111
Issuance of common stock in connection with at the market offering, net	13,015	$ 2	13,013
Issuance of common stock in connection with at the market offering, net (in shares)		2,814,673
Net loss	(57,204)			(57,204)
Balance at Sep. 30, 2020	144,918	$ 214	886,033	(741,329)
Balance (in shares) at Sep. 30, 2020		214,165,690
Balance at Jun. 30, 2020	163,414	$ 214	884,214	(721,014)
Balance (in shares) at Jun. 30, 2020		214,150,940
Stock-based compensation	1,792		1,792
Exercise of employee stock options	27		27
Exercise of employee stock options (in shares)		14,750
Net loss	(20,315)			(20,315)
Balance at Sep. 30, 2020	144,918	$ 214	886,033	(741,329)
Balance (in shares) at Sep. 30, 2020		214,165,690
Balance at Dec. 31, 2020	123,982	$ 215	887,868	(764,101)
Balance (in shares) at Dec. 31, 2020		214,591,906
Stock-based compensation	9,857		9,857
Exercise of employee stock options	$ 1,037		1,037
Exercise of employee stock options (in shares)	363,109	363,109
Restricted stock awards		$ 1	(1)
Restricted stock awards (in shares)		1,601,224
Issuance of warrants	$ 788		788
Cancelled restricted common stock	0	$ 0
Cancelled restricted common stock (in shares)		(410,435)
Net loss	(66,957)			(66,957)
Balance at Sep. 30, 2021	68,707	$ 216	899,549	(831,058)
Balance (in shares) at Sep. 30, 2021		216,145,804
Balance at Jun. 30, 2021	88,281	$ 216	896,390	(808,325)
Balance (in shares) at Jun. 30, 2021		215,559,148
Stock-based compensation	2,371		2,371
Restricted stock awards	0		0
Restricted stock awards (in shares)		875,000
Issuance of warrants	788		788
Cancelled restricted common stock (in shares)		(288,344)
Net loss	(22,733)			(22,733)
Balance at Sep. 30, 2021	$ 68,707	$ 216	$ 899,549	$ (831,058)
Balance (in shares) at Sep. 30, 2021		216,145,804