Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 17,975	$ 56,738	$ 37,059	$ 38,809	$ 39,106	$ 41,479	$ 31,792	$ 34,706	$ 150,581	$ 147,083	$ 137,959
Change in net unrealized gain (loss) on available-for-sale (AFS) securities, net of tax									4,093	(7,782)	(4,068)
Accretion of the net unrealized loss on the transfer of AFS securities to held-to-maturity (HTM) securities, net of tax									5	6	5
Change in defined benefit pension plan, net of tax									(1,406)	1,271	1,615
Comprehensive income, net of tax									$ 153,273	$ 140,578	$ 135,511