Condensed Consolidated Statements of Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (6,137)	$ (10,667)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available-for-sale investments, net of tax	20	(16)
Comprehensive loss	$ (6,117)	$ (10,683)