Condensed Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Gain/(Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2018	$ 149,122	$ 16	$ 275,593	$ (85)	$ (126,402)
Balance, shares at Dec. 31, 2018		28,823,130
Cumulative effect of adoption of Topic 842 | ASU 2016-02 [Member]	(153)				(153)
Unrealized (loss) gain on marketable securities, net of tax	152			152
Lapse of repurchase rights related to common stock issued pursuant to early exercises	17		17
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares		51,314
Issuance of common stock upon exercise of stock options	86		86
Issuance of common stock upon exercise of stock options, shares		149,938
Stock-based compensation	1,007		1,007
Net loss	(18,009)				(18,009)
Balance at Mar. 31, 2019	132,222	$ 16	276,703	67	(144,564)
Balance, shares at Mar. 31, 2019		29,024,382
Balance at Dec. 31, 2018	149,122	$ 16	275,593	(85)	(126,402)
Balance, shares at Dec. 31, 2018		28,823,130
Balance at Dec. 31, 2019	134,344	$ 17	355,291	24	(220,988)
Balance, shares at Dec. 31, 2019		36,332,956
Issuance of common stock under at the market ("ATM") equity offering program, net of issuance costs	5,577		5,577
Issuance of common stock under at the market ("ATM") equity offering program, net of issuance costs, shares		568,369
Unrealized (loss) gain on marketable securities, net of tax	(44)			(44)
Lapse of repurchase rights related to common stock issued pursuant to early exercises	8		8
Lapse of repurchase rights related to common stock issued pursuant to early exercises, shares		23,732
Issuance of common stock upon exercise of stock options	$ 91		91
Issuance of common stock upon exercise of stock options, shares	60,682	60,682
Stock-based compensation	$ 1,619		1,619
Net loss	(26,206)				(26,206)
Balance at Mar. 31, 2020	$ 115,389	$ 17	$ 362,586	$ (20)	$ (247,194)
Balance, shares at Mar. 31, 2020		36,985,739