Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 11,240	$ 11,126	$ 2,117	$ (101)
Unrealized gain (losses) on investments, net of tax	(975)	133	(4,939)	113
Comprehensive income (loss)	$ 10,265	$ 11,259	$ (2,822)	$ 12