CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS
Net loss	$ (102,496)	$ (88,720)	$ (75,738)
Unrealized gains (losses) on available-for-sale securities:
Unrealized gain (loss) on available-for-sale securities, net of tax	14	35	(17)
Comprehensive loss	$ (102,482)	$ (88,685)	$ (75,755)