Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$ 59,845us-gaap_NetIncomeLoss	$ 50,254us-gaap_NetIncomeLoss	$ 42,627us-gaap_NetIncomeLoss
Other comprehensive income (loss), net of tax
Net change in fair value of securities available for sale	5,412	(7,501)	(1,096)
Net change in fair value of cash flow hedges	2,256	3,735	1,082
Net change in other comprehensive income for defined benefit postretirement plans	(2,366)	858	(26)
Total other comprehensive income (loss)	5,302	(2,908)	(40)
Total comprehensive income	$ 65,147us-gaap_ComprehensiveIncomeNetOfTax	$ 47,346us-gaap_ComprehensiveIncomeNetOfTax	$ 42,587us-gaap_ComprehensiveIncomeNetOfTax

Net of tax amount of the appreciation (loss) in the value of unsold available-for-sale securities.

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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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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount after tax and reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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

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Amount after tax and reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

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