Condensed 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, 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	2,426				2,426
Shares issued upon exercise of stock options, Value	1,365		$ 1		1,364
Shares issued upon exercise of stock options, Shares			153,122
Net loss	(31,504)						(31,504)
Other comprehensive income (loss)	(16)							(16)
Balance, Value at Jun. 30, 2017	109,220		$ 33		302,920		(193,720)	(13)
Balance, Shares at Jun. 30, 2017			32,567,485
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
Sale of common stock under license agreement, Value	14,556		$ 1		14,555
Sale of common stock under license agreement, Shares			1,174,827
Stock-based compensation expense	3,940				3,940
Shares issued upon exercise of stock options, Value	1,748				1,748
Shares issued upon exercise of stock options, Shares			222,132
Net loss	(33,961)						(33,961)
Other comprehensive income (loss)	13							13
Balance, Value at Jun. 30, 2018	$ 73,076		$ 34		$ 327,401		$ (254,302)	$ (57)
Balance, Shares at Jun. 30, 2018			34,059,214