Consolidated Statement of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Comprehensive Income [Abstract]
Net income	$ 190,509	$ 188,692	$ 178,973
Investment securities available for sale:
Net unrealized gains (losses)	49,038	(158,207)	(19,042)
Reclassification adjustment for net (gains) losses realized in net income	(494)	(564)	(4,261)
Income tax (expense) benefit	(18,636)	61,266	9,651
Other comprehensive income (loss) on investment securities available for sale	29,908	(97,505)	(13,652)
Defined benefit pension and postretirement obligations:
Prior service cost, net of amortization	58	72	242
Net gain (loss), net of amortization	(17,079)	40,148	(6,941)
Income tax (expense) benefit	6,507	(15,562)	2,366
Other comprehensive income (loss) on pension and postretirement obligations	(10,514)	24,658	(4,333)
Derivatives used in cash flow hedging relationships:
Net unrealized gains (losses)	0	0	6
Reclassification adjustment for net losses and interest expense for interest differential on derivatives realized in net income	0	0	1,954
Income tax expense	0	0	(974)
Other comprehensive income on cash flow hedging relationships	0	0	986
Net other comprehensive income (loss) during period	19,394	(72,847)	(16,999)
Comprehensive income	$ 209,903	$ 115,845	$ 161,974