Consolidated Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Preferred Shares	Additional Paid-In Capital	Accumulated Deficit	ESPP Common Stock
Balance - Stockholders' Equity (Deficit) (in shares) at Dec. 31, 2018		10,847	701
Balance - Stockholders' Equity (Deficit) at Dec. 31, 2018	$ 7,335	$ 11		$ 298,627	$ (292,004)
Increase (Decrease) in Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity
Issuance of common stock (in shares)		3,200	0
Issuance of common stock	14,026	$ 3		14,023
Exercise of stock options (in shares)		3
Exercise of stock options	12			12
Stock-based compensation expense	429			429
Net loss	(15,567)				(15,567)
Balance - Stockholders' Equity (Deficit) (in shares) at Mar. 31, 2019		14,050	701
Balance - Stockholders' Equity (Deficit) at Mar. 31, 2019	6,235	$ 14		313,091	(307,571)
Balance - Stockholders' Equity (Deficit) (in shares) at Dec. 31, 2019		27,453	701
Balance - Stockholders' Equity (Deficit) at Dec. 31, 2019	25,042	$ 27		355,268	(330,954)
Increase (Decrease) in Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity
Issuance of common stock (in shares)		187				4
Issuance of common stock	440	$ 1		439
Stock-based compensation expense	384			384
Net loss	(12,853)				(12,853)
Balance - Stockholders' Equity (Deficit) (in shares) at Mar. 31, 2020		27,644	701
Balance - Stockholders' Equity (Deficit) at Mar. 31, 2020	$ 13,013	$ 28		$ 356,091	$ (343,807)