Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Follow-on Public Offering [Member]	Common Stock [Member]	Common Stock [Member] Follow-on Public Offering [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Follow-on Public Offering [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, Value at Dec. 31, 2014	$ 51,662		$ 23		$ 131,840		$ (80,201)
Balance, Shares at Dec. 31, 2014			22,802,039
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 75,231		$ 4		$ 75,227
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				4,327,956
Stock-based compensation expense	2,514				2,514
Shares issued upon exercise of stock options, Value	362				362
Shares issued upon exercise of stock options, Shares			124,868
Net loss	(24,690)						(24,690)
Other comprehensive income (loss)	(35)							$ (35)
Balance, Value at Dec. 31, 2015	105,044		$ 27		209,943		(104,891)	(35)
Balance, Shares at Dec. 31, 2015			27,254,863
Stock-based compensation expense	2,800				2,800
Shares issued upon exercise of stock options, Value	123				123
Shares issued upon exercise of stock options, Shares			42,000
Net loss	(57,280)						(57,280)
Other comprehensive income (loss)	38							38
Balance, Value at Dec. 31, 2016	50,725		$ 27		212,866		(162,171)	3
Balance, Shares at Dec. 31, 2016			27,296,863
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 86,224		$ 5		$ 86,219
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				5,117,500
Stock-based compensation expense	5,793				5,793
Modification of equity awards	537				537
Shares issued upon exercise of stock options, Value	1,699		$ 1		1,698
Shares issued upon exercise of stock options, Shares			247,892
Net loss	(58,125)						(58,125)
Other comprehensive income (loss)	(73)							(73)
Balance, Value at Dec. 31, 2017	$ 86,780		$ 33		307,158		(220,341)	$ (70)
Balance, Shares at Dec. 31, 2017			32,662,255
Cumulative effect adjustment upon adoption of ASU 2016-09					$ 45		$ (45)