Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2017		8,662,435
Beginning balance at Dec. 31, 2017	$ 35,216	$ 9	$ 381,199	$ (134)	$ (345,858)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		8,660
Stock-based compensation expense	1,627		1,627
Issuance of common stock upon exercise of options (in shares)		328
Issuance of common stock upon exercise of options	1		1
Issuance of common stock under Employee Stock Purchase Plan (in shares)		21,377
Issuance of common stock under Employee Stock Purchase Plan	207		207
Unrealized gain on short-term investments	7			7
Net loss	(16,025)				(16,025)
Ending balance (in shares) at Mar. 31, 2018		8,692,800
Ending balance at Mar. 31, 2018	22,876	$ 9	383,034	(127)	(360,040)
Beginning balance (in shares) at Dec. 31, 2018		8,818,019
Beginning balance at Dec. 31, 2018	(5,854)	$ 9	386,860	0	(392,723)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)		93,648
Stock-based compensation expense	959		959
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 upon sales using ATM	2,084	$ 2	2,082	0	0
Net loss	(3,260)				(3,260)
Ending balance (in shares) at Mar. 31, 2019		10,817,916
Ending balance at Mar. 31, 2019	$ (6,069)	$ 11	$ 389,903	$ 0	$ (395,983)