Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2017	$ 94,726	$ 52	$ 718,751	$ (624,077)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017		52,395,223
Issuance of common stock for exercise of stock options	101		101
Issuance of common stock for exercise of stock options, Shares		64,458
Stock-based compensation	1,499		1,499
Issuance of common stock, net of offering costs	45,996	$ 22	45,974
Issuance of common stock, net of offering costs, Shares		21,850,000
Net loss	(11,525)			(11,525)
Stockholders Equity, Ending Balance at Mar. 31, 2018	130,797	$ 74	766,325	(635,602)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2018		74,309,681
Stockholders Equity, Beginning Balance at Dec. 31, 2017	94,726	$ 52	718,751	(624,077)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017		52,395,223
Net loss	(23,485)
Stockholders Equity, Ending Balance at Jun. 30, 2018	120,651	$ 74	768,139	(647,562)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2018		74,334,181
Stockholders Equity, Beginning Balance at Mar. 31, 2018	130,797	$ 74	766,325	(635,602)
Stockholders Equity, Beginning Balance, Shares at Mar. 31, 2018		74,309,681
Issuance of common stock for exercise of stock options	32		32
Issuance of common stock for exercise of stock options, Shares		24,500
Stock-based compensation	1,782		1,782
Net loss	(11,960)			(11,960)
Stockholders Equity, Ending Balance at Jun. 30, 2018	120,651	$ 74	768,139	(647,562)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2018		74,334,181
Stockholders Equity, Beginning Balance at Dec. 31, 2018	94,479	$ 74	771,817	(677,412)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2018		74,365,965
Issuance of common stock for exercise of stock options	445		445
Issuance of common stock for exercise of stock options, Shares		322,318
Stock-based compensation	1,699		1,699
Net loss	(13,032)			(13,032)
Stockholders Equity, Ending Balance at Mar. 31, 2019	83,591	$ 74	773,961	(690,444)
Stockholders Equity, Ending Balance, Shares at Mar. 31, 2019		74,688,283
Stockholders Equity, Beginning Balance at Dec. 31, 2018	94,479	$ 74	771,817	(677,412)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2018		74,365,965
Net loss	(27,910)
Stockholders Equity, Ending Balance at Jun. 30, 2019	70,471	$ 74	775,719	(705,322)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2019		74,688,283
Stockholders Equity, Beginning Balance at Mar. 31, 2019	83,591	$ 74	773,961	(690,444)
Stockholders Equity, Beginning Balance, Shares at Mar. 31, 2019		74,688,283
Stock-based compensation	1,758		1,758
Net loss	(14,878)			(14,878)
Stockholders Equity, Ending Balance at Jun. 30, 2019	$ 70,471	$ 74	$ 775,719	$ (705,322)
Stockholders Equity, Ending Balance, Shares at Jun. 30, 2019		74,688,283