Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (17,322)	$ (1,673)	$ 900
Other comprehensive income (loss), net of tax:
Unrealized gains (losses) on investments	(26)	(10)
Total other comprehensive income (loss)	(26)	(10)
Comprehensive income (loss)	$ (17,348)	$ (1,683)	$ 900