Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ 177,604	$ 51	$ 733,907	$ (92)	$ (556,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	4,073		4,073
Exercise of stock options	13		13
Employee stock purchase plan purchases	170		170
Comprehensive loss:
Unrealized gain (loss) on investments, net	140			140
Net loss	(17,693)				(17,693)
Comprehensive loss	(17,553)
Ending Balance at Mar. 31, 2019	164,307	51	738,163	48	(573,955)
Beginning Balance at Dec. 31, 2019	109,952	62	778,693	35	(668,838)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,326		1,326
Employee stock purchase plan purchases	229		229
Comprehensive loss:
Unrealized gain (loss) on investments, net	(46)			(46)
Net loss	(10,420)				(10,420)
Comprehensive loss	(10,466)
Ending Balance at Mar. 31, 2020	$ 101,041	$ 62	$ 780,248	$ (11)	$ (679,258)