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	$ (89,490)	$ (60,058)	$ (51,802)
Unrealized gain (loss) on available-for-sale securities	26	(35)	22
Comprehensive loss	$ (89,464)	$ (60,093)	$ (51,780)