Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (13,420)	$ (14,333)	$ (25,093)	$ (30,694)
Other comprehensive income (loss):
Change in fair value of investments	189	20	382	15
Total change in unrealized gain on investments	189	20	382	15
Comprehensive loss	$ (13,231)	$ (14,313)	$ (24,711)	$ (30,679)