Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 67,064	$ 51,404	$ 269,345	$ 172,792
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	15,752	(1,577)	65,619	90,631
Tax effect	(5,857)	855	(24,401)	(34,250)
Reclassification adjustment for net gains realized and included in income before income taxes	(1,337)	(352)	(4,664)	(17,509)
Tax effect	495	191	1,726	6,583
Total change in unrealized gains and losses on securities, net of tax	9,053	(883)	38,280	45,455
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	0	0	1,429
Tax effect	0	0	0	(537)
Total change in unrecognized loss on cash flow hedges, net of tax	0	0	0	892
Change in pension obligation:
Amortization of actuarial losses and prior service cost	2,330	1,768	7,290	5,302
Tax effect	(864)	(642)	(2,702)	(1,993)
Total change in pension obligation, net of tax	1,466	1,126	4,588	3,309
Net current period other comprehensive income	10,519	243	42,868	49,656
Total comprehensive income	$ 77,583	$ 51,647	$ 312,213	$ 222,448