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, 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
Cumulative effect adjustment upon adoption of ASU 2016-09					45		(45)
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
Sale of common stock under license agreement, Value	14,556		$ 1		14,555
Sale of common stock under license agreement, Shares			1,174,827
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 92,063		$ 5		$ 92,058
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				5,175,000
Stock-based compensation expense	7,785				7,785
Modification of equity awards	616				616
Shares issued upon vesting of performance-based restricted stock units	1,693				1,693
Shares issued upon vesting of performance-based restricted stock units, Shares			83,791
Shares issued upon exercise of stock options, Value	4,194				4,194
Shares issued upon exercise of stock options, Shares			451,685
Net loss	(74,013)						(74,013)
Other comprehensive income (loss)	(44)							(44)
Balance, Value at Dec. 31, 2018	$ 133,630		$ 39		$ 428,059		$ (294,354)	$ (114)
Balance, Shares at Dec. 31, 2018			39,547,558