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	$ 19,802us-gaap_NetIncomeLoss	$ 17,029us-gaap_NetIncomeLoss
Unrealized gains and losses on securities available for sale:
Net unrealized gains arising during the period	3,711	3,719
Reclassification adjustment for (gains) losses included in net income	(1)	207
Total	3,710	3,926
Amortization related to post-retirement obligations	(4)	(48)
Total other comprehensive income	3,706	3,878
Total comprehensive income	$ 23,508us-gaap_ComprehensiveIncomeNetOfTax	$ 20,907us-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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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 of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.

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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 of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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

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