Investments (Aggregate Values and Unrealized Gains and Losses) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment securities available for sale
Cost	$ 7,011,477	$ 7,163,574
Gross Unrealized Gains	39,354	20,589
Gross Unrealized Losses	5,722	12,246
Fair Value	7,045,109us-gaap_AvailableForSaleSecurities	7,171,917us-gaap_AvailableForSaleSecurities
Investment securities held to maturity
Cost	441us-gaap_HeldToMaturitySecurities	518us-gaap_HeldToMaturitySecurities
U. S. Treasury
Investment securities available for sale
Cost	2,237,002	2,626,900
Gross Unrealized Gains	9,966	2,922
Gross Unrealized Losses	2	152
Fair Value	2,246,966	2,629,670
Government Agency
Investment securities available for sale
Cost	988,563	908,362
Gross Unrealized Gains	2,382	702
Gross Unrealized Losses	51	247
Fair Value	990,894	908,817
Mortgage Backed Securities, Other [Member]
Investment securities available for sale
Cost	3,785,912	3,628,187
Gross Unrealized Gains	27,006	16,964
Gross Unrealized Losses	5,669	11,847
Fair Value	3,807,249	3,633,304
Investment securities held to maturity
Cost	441	518
Held To Maturity Securities Accumulated Unrecognized Holding Gain	18	26
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	459	544
State, County and Municipal
Investment securities available for sale
Cost		125
Gross Unrealized Gains		1
Gross Unrealized Losses		0
Fair Value		$ 126

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

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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