Consolidated Statements of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Consolidated Statements of Comprehensive Income (Unaudited) [Abstract]
Net income per consolidated statements of income	$ 29,148us-gaap_NetIncomeLoss	$ 29,003us-gaap_NetIncomeLoss
Unrealized gains (losses) on available for sale securities and transferred securities:
Unrealized holding gains arising during the period	11,386	4,229
Less: adjustment for net gains realized in net income	0	(240)
Change in net unrealized holding loss on securities transferred to held to maturity	874	823
Pension and other postretirement benefit plans:
Net change in prior service costs	39	39
Recognized net loss due to lump sum settlements	257	232
Change in net actuarial loss	754	559
Derivatives:
Change in the accumulated gain on effective cash flow hedge derivatives	(608)	(408)
Less: adjustment for loss realized in net income	130	0
Other comprehensive income, net of tax	12,832	5,234
Comprehensive income	$ 41,980us-gaap_ComprehensiveIncomeNetOfTax	$ 34,237us-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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Net increase or decrease in the plan benefit obligation during the reporting period.

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