Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 19,073	$ 17,896	$ 16,639	$ 24,257	$ 23,073	$ 17,940	$ 14,319	$ 15,130	$ 77,865	$ 70,462	$ 53,666
Other comprehensive income:
Unrealized gain (loss) on securities, net of tax									(3,079)	(223)	7,105
Comprehensive income	$ 11,985	$ 16,183	$ 19,568	$ 27,051	$ 22,299	$ 19,248	$ 12,744	$ 15,948	$ 74,786	$ 70,239	$ 60,771