Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (29,497)	$ (42,854)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities	38	(43)
Total comprehensive loss	$ (29,459)	$ (42,897)