Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (5,329)	$ (65,025)	$ (26,887)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	5	(10)	9
Comprehensive loss	$ (5,324)	$ (65,035)	$ (26,878)