Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 51,404	$ 55,958	$ 172,792	$ 167,638
Unrealized gains and losses on securities:
Change in unrealized securities gains (losses) arising during period	(1,577)	28,231	90,631	29,420
Tax effect	855	(10,737)	(34,250)	(11,198)
Reclassification adjustment for net gains realized and included in income before income taxes	(352)	(5,564)	(17,509)	(10,837)
Tax effect	191	2,094	6,583	4,145
Total change in unrealized gains and losses on securities, net of tax	(883)	14,024	45,455	11,530
Change in fair value of cash flow hedges:
Change in unrecognized loss on cash flow hedges	0	721	1,429	2,006
Tax effect	0	(300)	(537)	(796)
Total change in unrecognized loss on cash flow hedges, net of tax	0	421	892	1,210
Change in pension obligation:
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax	1,768	2,916	5,302	8,689
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax	(642)	(1,078)	(1,993)	(3,324)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	1,126	1,838	3,309	5,365
Net current period other comprehensive (loss) income	243	16,283	49,656	18,105
Total comprehensive income	$ 51,647	$ 72,241	$ 222,448	$ 185,743