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 (loss)	$ 29,988us-gaap_NetIncomeLoss	$ 28,787us-gaap_NetIncomeLoss
Unrealized gains (losses) on investments available-for-sale:
Unrealized holding gains arising during the period	16,282	28,388
Less: Reclassification adjustments for gains included in earnings	(605)	(308)
Unrealized gains on investments available-for-sale, net	15,677	28,080
Defined benefit pension plans:
Prior service cost arising during the period	0	0
Net loss arising during the period	0	0
Less: Amortization of prior service cost and net loss included in net periodic pension cost	640	150
Defined benefit pension plans, net	640	150
Other comprehensive income before tax	16,317	28,230
Income tax expense related to components of other comprehensive income	5,758us-gaap_OtherComprehensiveIncomeLossTax	9,873us-gaap_OtherComprehensiveIncomeLossTax
Other comprehensive income, net of tax	10,559	18,357
Comprehensive income	$ 40,547us-gaap_ComprehensiveIncomeNetOfTax	$ 47,144us-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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Amount before tax of cost (credit) of benefit changes attributable to plan participants' prior service resulting from a plan amendment or plan initiation that has not been recognized in net periodic benefit cost (credit).

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