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	$ (21,561)	$ (16,945)
Other comprehensive income (loss):
Unrealized (gain) loss on available-for-sale securities	1	(2)
Total comprehensive loss	$ (21,560)	$ (16,947)