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Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2014 | Dec. 31, 2013
--- | --- | ---
Held-to-maturity securities |  |
Amortized Cost | $ 2,400 | $ 2,400
Estimated Fair Value | 2,400 | 2,400
Carrying Value | 2,400 | 2,400
Available-for-sale securities |  |
Amortized Cost | 5,209,023 | 5,372,594
Gross Unrealized Gains | 76,469 | 65,104
Gross Unrealized Losses | (60,449) | (133,119)
Total investment securities | 5,225,043 | 5,304,579
Estimate Of Fair Value Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 5,225,043 | 5,304,579
Carrying Reported Amount Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 5,225,043 | 5,304,579
Other Debt Securities [Member] |  |
Held-to-maturity securities |  |
Amortized Cost | 2,400 | 2,400
Estimated Fair Value | 2,400 | 2,400
Carrying Value | 2,400 | 2,400
Mortgage Backed Securities [Member] |  |
Available-for-sale securities |  |
Amortized Cost | 4,923,587 | 5,096,165
Gross Unrealized Gains | 58,667 | 56,110
Gross Unrealized Losses | (54,391) | (124,574)
Mortgage Backed Securities [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 4,927,863 | 5,027,701
Mortgage Backed Securities [Member] | Carrying Reported Amount Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 4,927,863 | 5,027,701
U S Government Corporations And Agencies Securities [Member] |  |
Available-for-sale securities |  |
Total investment securities | 1,646,495 | 1,799,807
Mortgage Backed Securities Issued By U S Government Sponsored Enterprises [Member] |  |
Available-for-sale securities |  |
Total investment securities | 3,255,372 | 3,200,042
Mortgage Backed Securities Issued By Private Enterprises [Member] |  |
Available-for-sale securities |  |
Total investment securities | 25,996 | 27,852
U S States And Political Subdivisions [Member] |  |
Available-for-sale securities |  |
Amortized Cost | 257,361 | 248,354
Gross Unrealized Gains | 16,550 | 8,063
Gross Unrealized Losses | (5,817) | (8,007)
U S States And Political Subdivisions [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 268,094 | 248,410
U S States And Political Subdivisions [Member] | Carrying Reported Amount Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 268,094 | 248,410
Equity Securities [Member] |  |
Available-for-sale securities |  |
Amortized Cost | 28,075 | 28,075
Gross Unrealized Gains | 1,252 | 931
Gross Unrealized Losses | (241) | (538)
Equity Securities [Member] | Estimate Of Fair Value Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | 29,086 | 28,468
Equity Securities [Member] | Carrying Reported Amount Fair Value Disclosure [Member] |  |
Available-for-sale securities |  |
Total investment securities | $ 29,086 | $ 28,468

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