Investments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Amortized Cost Basis [Abstract]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 18,358
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	76,173
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	276,933
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	1,175,551
Available-for-sale Securities, Amortized Cost Basis	1,547,015	1,522,871
Fair Value Disclosures [Abstract]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	18,538
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	76,515
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	287,023
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	1,213,333
Available-for-sale Securities	$ 1,595,409us-gaap_AvailableForSaleSecurities	$ 1,567,177us-gaap_AvailableForSaleSecurities

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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Amount of available-for-sale debt securities at fair value maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year.

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Amount of available-for-sale debt securities at cost, net of adjustments, maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings (less any cumulative-effect adjustments, as defined) and fair value hedge accounting adjustments.

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Amount of available-for-sale debt securities at fair value maturing in the second fiscal year through the fifth fiscal year following the latest fiscal year.

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