Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 15,240us-gaap_NetIncomeLoss	$ 13,597us-gaap_NetIncomeLoss
Securities:
Net change in unrealized holding gains on securities	2,624	2,784
Amortization of unrealized holding losses on securities transferred to the held to maturity category	(32)	(44)
Total securities	2,592	2,740
Derivative instruments:
Net change in unrealized holding losses on derivative instruments	(669)	(419)
Totals derivative instruments	(669)	(419)
Defined benefit pension and post-retirement benefit plans:
Amortization of net actuarial loss recognized in net periodic pension cost	57	45
Total defined benefit pension and post-retirement benefit plans	57	45
Other comprehensive income, net of tax	1,980	2,366
Comprehensive income	$ 17,220us-gaap_ComprehensiveIncomeNetOfTax	$ 15,963us-gaap_ComprehensiveIncomeNetOfTax

Amortization of unrealized holding gains on securities transferred to the held to maturity category.

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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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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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