Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net loss	$ (43,562)	$ (47,089)	$ (45,581)
Other comprehensive income (loss):
Change in unrealized gain (loss) on short-term investments	(42)	(28)	14
Comprehensive loss	$ (43,604)	$ (47,117)	$ (45,567)