Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 25,170	$ 18,799	$ 25,494
Unrealized (losses) gains on investments, net of tax	(1,427)	28	(8)
Comprehensive income	$ 23,743	$ 18,827	$ 25,486