CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (26,141)	$ (29,309)	$ (32,099)
Other comprehensive income/(loss)
Foreign currency translation adjustments, net	261	2,210	561
Unrealized gain on available-for-sale securities			217
Comprehensive loss	$ (25,880)	$ (27,099)	$ (31,321)