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	$ 157,894us-gaap_NetIncomeLoss	$ 161,840us-gaap_NetIncomeLoss	$ 159,845us-gaap_NetIncomeLoss
Unrealized gain (loss) on securities	33,734	(49,607)	1,569
Reclassification adjustment for securities gains included in net income	(1,327)	(5,203)	(1,967)
Reclassification adjustment for postretirement plan curtailment gain included in net income	(944)	0	0
Non-credit related unrealized gain on other-than-temporarily impaired debt securities	780	1,977	1,330
Unrealized gain on derivative financial instruments	136	136	136
Unrecognized pension and postretirement (cost) income	(13,168)	8,369	(4,207)
Amortization of net unrecognized pension and postretirement income	408	1,312	859
Other Comprehensive Income (Loss)	19,619	(43,016)	(2,280)
Total Comprehensive Income	$ 177,513us-gaap_ComprehensiveIncomeNetOfTax	$ 118,824us-gaap_ComprehensiveIncomeNetOfTax	$ 157,565us-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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