Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2017	$ 117,279	$ 31	$ 482,198	$ (100)	$ (364,850)
Beginning balance, shares at Dec. 31, 2017		30,558,726
Issuance of common stock upon exercise of stock options (unaudited)	32		32
Issuance of common stock upon exercise of stock options (unaudited), shares		18,800
Stock-based compensation expense (unaudited)	2,713		2,713
Net loss (unaudited)	(11,424)				(11,424)
Unrealized gain on available- for-sale securities (unaudited)	4			4
Ending balance (unaudited) at Mar. 31, 2018	108,604	$ 31	484,943	(96)	(376,274)
Ending balance (unaudited), shares at Mar. 31, 2018		30,577,526
Beginning balance at Dec. 31, 2018	$ 79,737	$ 31	494,947	(23)	(415,218)
Beginning balance, shares at Dec. 31, 2018		30,685,412
Issuance of common stock upon exercise of stock options (unaudited), shares	0
Stock-based compensation expense (unaudited)	$ 1,496		1,496
Net loss (unaudited)	(11,984)				(11,984)
Unrealized gain on available- for-sale securities (unaudited)	40			40
Ending balance (unaudited) at Mar. 31, 2019	$ 69,289	$ 31	$ 496,443	$ 17	$ (427,202)
Ending balance (unaudited), shares at Mar. 31, 2019		30,685,412