Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Common Stock [Member] Follow-on Public Offering [Member]	Additional Paid-In Capital [Member]	Subscriptions Receivable [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, Value at Dec. 31, 2016	$ 50,725	$ 27		$ 212,866		$ (162,171)	$ 3
Balance, Shares at Dec. 31, 2016		27,296,863
Subscription of common stock, net of underwriting discounts and commissions and offering expenses, Value	(530)		$ 5	85,983	$ (86,518)
Subscription of common stock, net of underwriting discounts and commissions and offering expenses, Shares			5,117,500
Stock-based compensation expense	1,108			1,108
Shares issued upon exercise of stock options, Value	149			149
Shares issued upon exercise of stock options, Shares		27,291
Net loss	(22,204)					(22,204)
Other comprehensive income (loss)	21						21
Balance, Value at Mar. 31, 2017	29,269	$ 27	$ 5	300,151	$ (86,518)	(184,420)	24
Balance, Shares at Mar. 31, 2017		27,324,154	5,117,500
Cumulative effect adjustment upon adoption of ASU 2016-09				45		(45)
Balance, Value at Dec. 31, 2017	86,780	$ 33		307,158		(220,341)	(70)
Balance, Shares at Dec. 31, 2017		32,662,255
Stock-based compensation expense	1,871			1,871
Shares issued upon exercise of stock options, Value	263			263
Shares issued upon exercise of stock options, Shares		37,688
Net loss	(16,767)					(16,767)
Other comprehensive income (loss)	(44)						(44)
Balance, Value at Mar. 31, 2018	$ 72,103	$ 33		$ 309,292		$ (237,108)	$ (114)
Balance, Shares at Mar. 31, 2018		32,699,943