STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($)	Total	Common Stock	Additional Paid In Capital	Accumulated Deficit
Balance at Dec. 31, 2019	$ 95,010,000	$ 182,000	$ 778,953,000	$ (684,125,000)
Balance (in shares) at Dec. 31, 2019		181,803,320
Stock-based compensation	3,601,000		3,601,000
Exercise of employee stock options	16,000		16,000
Exercise of employee stock options (in shares)		8,166
Issuance of restricted common stock	0	$ 1,000	(1,000)
Issuance of restricted common stock (in shares)		555,900
Issuance of common stock in connection with a public offering, net	88,661,000	$ 29,000	88,632,000
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	13,015,000	$ 2,000	13,013,000
Issuance of common stock in connection with at the market offering, net (in shares)		2,814,673
Net loss	(36,889,000)			(36,889,000)
Balance at Jun. 30, 2020	163,414,000	$ 214,000	884,214,000	(721,014,000)
Balance (in shares) at Jun. 30, 2020		214,292,170
Balance at Mar. 31, 2020	180,337,000	$ 214,000	882,541,000	(702,418,000)
Balance (in shares) at Mar. 31, 2020		214,286,337
Stock-based compensation	1,661,000		1,661,000
Exercise of employee stock options	12,000		12,000
Exercise of employee stock options (in shares)		5,833
Net loss	(18,596,000)			(18,596,000)
Balance at Jun. 30, 2020	163,414,000	$ 214,000	884,214,000	(721,014,000)
Balance (in shares) at Jun. 30, 2020		214,292,170
Balance at Dec. 31, 2020	123,982,000	$ 215,000	887,868,000	(764,101,000)
Balance (in shares) at Dec. 31, 2020		214,591,906
Stock-based compensation	7,486,000		7,486,000
Exercise of employee stock options	$ 1,036,000	$ 0	1,036,000
Exercise of employee stock options (in shares)	363,109	363,109
Restricted stock awards	$ 1,000	$ 1,000	0
Restricted stock awards (in shares)		726,224
Cancelled restricted common stock	0	$ 0
Cancelled restricted common stock (in shares)		122,091
Net loss	(44,224,000)			(44,224,000)
Balance at Jun. 30, 2021	88,281,000	$ 216,000	896,390,000	(808,325,000)
Balance (in shares) at Jun. 30, 2021		215,559,148
Balance at Mar. 31, 2021	105,641,000	$ 215,000	891,081,000	(785,655,000)
Balance (in shares) at Mar. 31, 2021		215,257,674
Stock-based compensation	5,290,000		5,290,000
Exercise of employee stock options	20,000		20,000
Exercise of employee stock options (in shares)		10,667
Restricted stock awards	0	$ 1,000	1,000
Restricted stock awards (in shares)		412,898
Cancelled restricted common stock (in shares)		122,091
Net loss	(22,670,000)			(22,670,000)
Balance at Jun. 30, 2021	$ 88,281,000	$ 216,000	$ 896,390,000	$ (808,325,000)
Balance (in shares) at Jun. 30, 2021		215,559,148