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	$ (45,470)	$ (49,000)	$ (49,857)
Other comprehensive loss:
Change in fair value of investments	(23)	(19)	0
Adjustment for net losses realized and included in net loss	3	0	0
Total change in unrealized losses on investments	(20)	(19)	0
Comprehensive loss	$ (45,490)	$ (49,019)	$ (49,857)