CONDENSED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated deficit	Convertible Preferred Stock [Member]
Beginning balance (in shares) at Dec. 31, 2018		8,818,019			0
Beginning balance at Dec. 31, 2018	$ (5,854)	$ 9	$ 386,860	$ (392,723)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		93,648
Issuance of common stock under Employee Stock Purchase Plan (in shares)		2,369
Issuance of common stock under Employee Stock Purchase Plan	2		2
Issuance of common stock upon sales using ATM (in shares)		1,903,880
Issuance of common stock through ATM	2,084	$ 2	2,082
Stock-based compensation expense	959		959
Net loss	(3,260)			(3,260)
Ending balance (in shares) at Mar. 31, 2019		10,817,916			0
Ending balance at Mar. 31, 2019	(6,069)	$ 11	389,903	(395,983)	$ 0
Beginning balance (in shares) at Dec. 31, 2019		21,018,663			3,704,288
Beginning balance at Dec. 31, 2019	20,015	$ 21	431,305	(411,315)	$ 4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		21,166
Issuance of common stock upon exercise of options (in shares)		468
Issuance of common stock under Employee Stock Purchase Plan (in shares)		1,666
Issuance of common stock under Employee Stock Purchase Plan	1		1
Conversions of convertible preferred stock (in shares)		6,566,820			(656,682)
Conversions of convertible preferred stock		$ 1			$ (1)
Stock-based compensation expense	823		823
Net loss	(5,937)			(5,937)
Ending balance (in shares) at Mar. 31, 2020		27,608,783			3,047,606
Ending balance at Mar. 31, 2020	$ 14,902	$ 22	$ 432,129	$ (417,252)	$ 3