Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
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
Balance, Value at Dec. 31, 2017	86,780	$ 33	307,158	(220,341)	(70)
Balance, Shares at Dec. 31, 2017		32,662,255
Net loss	(33,961)
Other comprehensive income (loss)	13
Balance, Value at Jun. 30, 2018	73,076	$ 34	327,401	(254,302)	(57)
Balance, Shares at Jun. 30, 2018		34,059,214
Balance, Value at Mar. 31, 2018	72,103	$ 33	309,292	(237,108)	(114)
Balance, Shares at Mar. 31, 2018		32,699,943
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	2,069		2,069
Shares issued upon exercise of stock options, Value	1,485		1,485
Shares issued upon exercise of stock options, Shares		184,444
Net loss	(17,194)			(17,194)
Other comprehensive income (loss)	57				57
Balance, Value at Jun. 30, 2018	73,076	$ 34	327,401	(254,302)	(57)
Balance, Shares at Jun. 30, 2018		34,059,214
Balance, Value at Dec. 31, 2018	133,630	$ 39	428,059	(294,354)	(114)
Balance, Shares at Dec. 31, 2018		39,547,558
Stock-based compensation expense	2,234		2,234
Shares issued upon exercise of stock options, Value	234		234
Shares issued upon exercise of stock options, Shares		17,291
Shares issued for consulting services	197		197
Shares issued for consulting services, Shares		10,195
Net loss	(21,960)			(21,960)
Other comprehensive income (loss)	187				187
Balance, Value at Mar. 31, 2019	114,522	$ 39	430,724	(316,314)	73
Balance, Shares at Mar. 31, 2019		39,575,044
Balance, Value at Dec. 31, 2018	133,630	$ 39	428,059	(294,354)	(114)
Balance, Shares at Dec. 31, 2018		39,547,558
Net loss	(44,920)
Other comprehensive income (loss)	279
Balance, Value at Jun. 30, 2019	99,545	$ 40	438,614	(339,274)	165
Balance, Shares at Jun. 30, 2019		40,027,916
Balance, Value at Mar. 31, 2019	114,522	$ 39	430,724	(316,314)	73
Balance, Shares at Mar. 31, 2019		39,575,044
Stock-based compensation expense	2,681		2,681
Shares issued upon exercise of stock options, Value	3,975	$ 1	3,974
Shares issued upon exercise of stock options, Shares		378,706
Shares issued upon vesting of restricted stock units	1,235		1,235
Shares issued upon vesting of restricted stock units, Shares		74,166
Net loss	(22,960)			(22,960)
Other comprehensive income (loss)	92				92
Balance, Value at Jun. 30, 2019	$ 99,545	$ 40	$ 438,614	$ (339,274)	$ 165
Balance, Shares at Jun. 30, 2019		40,027,916