Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)	Total
Beginning Balance at Dec. 31, 2016	$ 56	$ 364,148	$ (285,625)	$ 2	$ 78,581
Balance (in shares) at Dec. 31, 2016	55,768,414
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		6,387			6,387
Exercise of stock options		361			361
Exercise of stock options (in shares)	293,809
Issuance of warrants		501			501
Issuance of common stock, net of issuance costs	$ 6	20,706			20,712
Issuance of common stock, net of issuance costs (in shares)	6,248,572
Unrealized gain (loss) on marketable securities				(44)	(44)
Net loss			(71,865)		(71,865)
Ending Balance at Dec. 31, 2017	$ 62	392,103	(357,490)	(42)	34,633
Balance (in shares) at Dec. 31, 2017	62,310,795
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		4,367			4,367
Exercise of stock options		83			83
Exercise of stock options (in shares)	133,074
Issuance of common stock, net of issuance costs	$ 20	33,174			33,194
Issuance of common stock, net of issuance costs (in shares)	19,879,544
Unrealized gain (loss) on marketable securities				33	33
Net loss			(30,784)		(30,784)
Ending Balance at Dec. 31, 2018	$ 82	429,727	(388,274)	(9)	$ 41,526
Balance (in shares) at Dec. 31, 2018	82,323,413				82,323,413
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		3,232			$ 3,232
Exercise of stock options		28			28
Exercise of stock options (in shares)	41,968
Issuance of warrants		347			347
Issuance of common stock, net of issuance costs	$ 12	9,953			9,965
Issuance of common stock, net of issuance costs (in shares)	11,442,408
Shares repurchased by the Company (in shares)	(122)
Issuance of common stock upon vesting of RSUs, net of shares withheld for employee taxes		(158)			(158)
Issuance of common stock upon vesting of RSUs, net of shares withheld for employee taxes (in shares)	406,093
Unrealized gain (loss) on marketable securities				$ 9	9
Net loss			(24,871)		(24,871)
Ending Balance at Dec. 31, 2019	$ 94	$ 443,129	$ (413,145)		$ 30,078
Balance (in shares) at Dec. 31, 2019	94,213,760				94,213,760