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	4,046				4,046
Modification of equity awards	474				474
Shares issued upon exercise of stock options, Value	1,516		$ 1		1,515
Shares issued upon exercise of stock options, Shares			194,122
Cumulative effect adjustment upon adoption of ASU 2016-09					45		(45)
Net loss	(43,948)						(43,948)
Other comprehensive income	37							37
Balance, Value at Sep. 30, 2017	99,074		$ 33		305,165		(206,164)	40
Balance, Shares at Sep. 30, 2017			32,608,485
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, net of underwriting discounts and commissions and offering expenses, Value		$ 92,077		$ 5		$ 92,072
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				5,175,000
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	5,759				5,759
Shares issued upon exercise of stock options, Value	3,636				3,636
Shares issued upon exercise of stock options, Shares			377,433
Net loss	(53,361)						(53,361)
Other comprehensive income	83							83
Balance, Value at Sep. 30, 2018	$ 149,530		$ 39		$ 423,180		$ (273,702)	$ 13
Balance, Shares at Sep. 30, 2018			39,389,515