Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 323,752	$ 225,482	$ 210,386
Unrealized gains (losses) on securities:
Change in unrealized securities gains (losses) arising during period	28,166	(21,530)	(22,030)
Tax effect	(10,531)	7,584	8,486
Reclassification adjustment for net gains realized and included in income before income taxes	(4,293)	(26,673)	(10,817)
Tax effect	1,588	9,869	4,138
Total change in unrealized gains (losses) on securities, net of tax	14,930	(30,750)	(20,223)
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	1,429	2,908
Tax effect	0	(537)	(1,136)
Total change in unrecognized loss on cash flow hedges, net of tax	0	892	1,772
Change in pension obligation:
Change in pension obligation	(12,945)	(70,424)	691
Tax effect	4,789	25,077	(297)
Amortization of actuarial losses and prior service cost	9,720	7,069	11,586
Tax effect	(3,596)	(2,616)	(4,988)
Total change in pension obligation, net of tax	(2,032)	(40,894)	6,992
Net current period other comprehensive income (loss)	12,898	(70,752)	(11,459)
Total comprehensive income	$ 336,650	$ 154,730	$ 198,927