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	$ 57,910us-gaap_NetIncomeLoss	$ 50,539us-gaap_NetIncomeLoss	$ 34,200us-gaap_NetIncomeLoss
Other Comprehensive Income (Loss), Before Tax:
Net change in unrealized gains (losses) on securities available-for-sale	11,825	(16,928)	4,097
Net available-for-sale securities gains reclassified into earnings	(41)	(5)	(3,016)
Adjustment to funded status of employee benefit plans	(13,394)	18,299	(271)
Other Comprehensive Income (Loss), Before Tax	(1,610)	1,366	810
Income tax benefit (expense) related to items of other comprehensive income	471	(478)	(284)
Other Comprehensive Income (Loss), After Tax	(1,139)	888	526
Comprehensive Income	$ 56,771us-gaap_ComprehensiveIncomeNetOfTax	$ 51,427us-gaap_ComprehensiveIncomeNetOfTax	$ 34,726us-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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