Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance, Value at Dec. 31, 2016	$ 27	$ 212,866	$ (162,171)	$ 3	$ 50,725
Balance, Shares at Dec. 31, 2016	27,296,863
Sale of common stock, net of underwriting discounts and commissions and offering expenses, value	$ 5	86,219			86,224
Sale of common, net of underwriting discounts and commissions and offering expenses, shares	5,117,500
Stock-based compensation expense		5,793			5,793
Shares issued upon exercise of stock options, Value	$ 1	1,698			1,699
Shares issued upon exercise of stock options, Shares	247,892
Modification of equity awards		537			537
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	$ 33	307,158	(220,341)	(70)	86,780
Balance, Shares at Dec. 31, 2017	32,662,255
Sale of common stock under license agreement, Value	$ 1	14,555			14,556
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	$ 5	92,058			92,063
Sale of common, net of underwriting discounts and commissions and offering expenses, shares	5,175,000
Stock-based compensation expense		7,785			7,785
Shares issued upon exercise of stock options, Value		4,194			4,194
Shares issued upon exercise of stock options, Shares	451,685
Shares issued upon vesting of executive restricted stock units		1,693			1,693
Shares issued upon vesting of executive restricted stock units, Shares	83,791
Modification of equity awards		616			616
Net loss			(74,013)		(74,013)
Other comprehensive income (loss)				(44)	(44)
Balance, Value at Dec. 31, 2018	$ 39	428,059	(294,354)	(114)	133,630
Balance, Shares at Dec. 31, 2018	39,547,558
Sale of common stock under license agreement, Value		4,000			4,000
Sale of common stock under license agreement, Shares	170,793
Sale of common stock, net of underwriting discounts and commissions and offering expenses, value	$ 7	136,491			136,498
Sale of common, net of underwriting discounts and commissions and offering expenses, shares	6,325,000
Stock-based compensation expense		10,587			10,587
Shares issued upon exercise of stock options, Value	$ 1	6,105			$ 6,106
Shares issued upon exercise of stock options, Shares	555,847				555,847
Shares issued for consulting services		197			$ 197
Shares issued for consulting services, Shares	10,195
Shares issued upon vesting of executive restricted stock units		1,784			1,784
Shares issued upon vesting of executive restricted stock units, Shares	110,832
Net loss			(106,373)		(106,373)
Other comprehensive income (loss)				284	284
Balance, Value at Dec. 31, 2019	$ 47	$ 587,223	$ (400,727)	$ 170	$ 186,713
Balance, Shares at Dec. 31, 2019	46,720,225