Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 46,672us-gaap_NetIncomeLoss	$ 45,199us-gaap_NetIncomeLoss
Investment securities available for sale:
Net unrealized gains	47,418	20,627
Reclassification adjustment for net gains realized in net income	0	(378)
Income tax expense	(18,105)	(7,786)
Other comprehensive income on investment securities available for sale	29,313	12,463
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	13	15
Amortization of actuarial losses	532	316
Income tax expense	(208)	(127)
Other comprehensive income on pension and postretirement obligations	337	204
Total other comprehensive income	29,650	12,667
Comprehensive income	$ 76,322us-gaap_ComprehensiveIncomeNetOfTax	$ 57,866us-gaap_ComprehensiveIncomeNetOfTax

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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