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, 2017	$ 33	$ 307,158	$ (220,341)	$ (70)	$ 86,780
Balance, Shares at Dec. 31, 2017	32,662,255
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, value	$ 5	92,058			92,063
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, Shares	5,175,000
Sale of common stock under license agreement, value	$ 1	14,555			14,556
Sale of common stock under license agreement, shares	1,174,827
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 restricted stock units, value		1,693			1,693
Shares issued upon vesting of restricted stock units, shares	83,791
Modification of equity awards		616			616
Net income (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 in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, value	$ 7	136,491			136,498
Sale of common stock in a follow-on public offering, net of underwriting discounts and commissions and offering expenses, Shares	6,325,000
Sale of common stock under license agreement, value		4,000			4,000
Sale of common stock under license agreement, shares	170,793
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
Shares issued upon vesting of restricted stock units, value		1,784			1,784
Shares issued upon vesting of restricted stock units, shares	110,832
Shares issued for consulting services, value		197			197
Shares issued for consulting services, shares	10,195
Net income (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
Sale of common stock under license agreement, value	$ 3	38,446			38,449
Sale of common stock under license agreement, shares	2,939,552
Stock-based compensation expense		12,486			12,486
Shares issued upon exercise of stock options, value		691			$ 691
Shares issued upon exercise of stock options, shares	55,852				55,852
Shares issued upon vesting of restricted stock units, value		2,349			$ 2,349
Shares issued upon vesting of restricted stock units, shares	156,584
Net income (loss)			8,410		8,410
Other comprehensive income (loss)				(97)	(97)
Balance, Value at Dec. 31, 2020	$ 50	$ 641,195	$ (392,317)	$ 73	$ 249,001
Balance, Shares at Dec. 31, 2020	49,872,213