STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Accumulated Deficit
Balance at Dec. 31, 2018	$ 85,564	$ 161	$ 651,732	$ (566,329)
Balance (in shares) at Dec. 31, 2018		161,066,136
Stock-based compensation	3,255		3,255
Exercise of employee stock options	600		600
Exercise of employee stock options (in shares)		258,068
Issuance of restricted common stock	1,000	$ 1	999
Issuance of restricted common stock (in shares)		1,393,536
Cancelled restricted common stock (in shares)		(74,599)
Repurchase of restricted common stock	(370)		(370)
Repurchase of restricted common stock (in shares)		(165,178)
Net loss	(28,054)			(28,054)
Balance at Jun. 30, 2019	61,995	$ 162	656,216	(594,383)
Balance (in shares) at Jun. 30, 2019		162,477,963
Balance at Mar. 31, 2019	74,216	$ 162	653,817	(579,763)
Balance (in shares) at Mar. 31, 2019		162,287,161
Stock-based compensation	1,799		1,799
Exercise of employee stock options	600		600
Exercise of employee stock options (in shares)		258,068
Cancelled restricted common stock (in shares)		(67,266)
Net loss	(14,620)			(14,620)
Balance at Jun. 30, 2019	61,995	$ 162	656,216	(594,383)
Balance (in shares) at Jun. 30, 2019		162,477,963
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	3,601		3,601
Exercise of employee stock options	$ 16		16
Exercise of employee stock options (in shares)	8,166	8,166
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	(36,889)			(36,889)
Balance at Jun. 30, 2020	163,414	$ 214	884,214	(721,014)
Balance (in shares) at Jun. 30, 2020		214,292,170
Balance at Mar. 31, 2020	180,337	$ 214	882,541	(702,418)
Balance (in shares) at Mar. 31, 2020		214,286,337
Stock-based compensation	1,661		1,661
Exercise of employee stock options	12		12
Exercise of employee stock options (in shares)		5,833
Net loss	(18,596)			(18,596)
Balance at Jun. 30, 2020	$ 163,414	$ 214	$ 884,214	$ (721,014)
Balance (in shares) at Jun. 30, 2020		214,292,170