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	1,456		1,456
Issuance of restricted common stock		$ 1	999	1,000
Issuance of restricted common stock (in shares)		1,393,536
Cancelled restricted common stock (in shares)		(7,333)
Repurchase of restricted common stock	(370)		(370)
Repurchase of restricted common stock (Shares)		(165,178)
Net loss	(13,434)			(13,434)
Balance at Mar. 31, 2019	74,216	$ 162	653,817	(579,763)
Balance (in shares) at Mar. 31, 2019		162,287,161
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	1,940		1,940
Exercise of employee stock options	$ 4		4
Exercise of employee stock options (in shares)	2,333	2,333
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	(18,293)			(18,293)
Balance at Mar. 31, 2020	$ 180,337	$ 214	$ 882,541	$ (702,418)
Balance (in shares) at Mar. 31, 2020		214,286,337