Investments (Maturity Information) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,011,477	$ 7,163,574
Total investment securities available for sale, fair value	7,045,109us-gaap_AvailableForSaleSecurities	7,171,917us-gaap_AvailableForSaleSecurities
Investment securities held to maturity
Investment securities held to maturity	441us-gaap_HeldToMaturitySecurities	518us-gaap_HeldToMaturitySecurities
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	504,826	447,866
Maturing in one through five years, cost	2,720,739	3,087,521
Maturing in one year or less, fair value	505,182	447,992
Maturing in one through five years, fair value	2,732,678	3,090,621
Mortgage Backed Securities, Other [Member]
Investment securities available for sale
Total investment securities available for sale, cost	3,785,912	3,628,187
Total investment securities available for sale, fair value	3,807,249	3,633,304
Investment securities held to maturity
Investment securities held to maturity	441	518
Fair Value	$ 459	$ 544

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

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