Investments (Maturity Information) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,163,574	$ 5,404,335
Investment securities available for sale	7,171,917us-gaap_AvailableForSaleSecurities	5,387,703us-gaap_AvailableForSaleSecurities
Investment securities held to maturity
Investment securities held to maturity	518us-gaap_HeldToMaturitySecurities	907us-gaap_HeldToMaturitySecurities
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	447,866	839,956
Maturing in one through five years, cost	3,087,521	2,077,539
Maturing in one year or less, fair value	447,992	840,883
Maturing in one through five years, fair value	3,090,621	2,077,800
Equity Securities
Investment securities available for sale
Total investment securities available for sale, cost	0	543
Investment securities available for sale	0	22,147
Mortgage Backed Securities, Other [Member]
Investment securities available for sale
Total investment securities available for sale, cost	3,628,187	2,486,297
Investment securities available for sale	3,633,304	2,446,873
Investment securities held to maturity
Investment securities held to maturity	518	907
Fair Value	$ 544	$ 974

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

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