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Investment Securities (Details) (USD $) In Thousands | Sep. 30, 2011 | Dec. 31, 2010
--- | --- | ---
Held-to-maturity securities |  |
Amortized cost | $ 2,575 | $ 2,450
Estimated fair value | 2,575 | 2,450
Carrying value | 2,575 | 2,450
Available-for-sale securities |  |
Amortized cost | 4,840,723 | 5,041,847
Gross unrealized gains | 160,308 | 79,241
Gross unrealized Losses | (12,118) | (34,631)
Estimated fair value | 4,988,913 | 5,086,457
Carrying value | 4,988,913 | 5,086,457
Other securities |  |
Held-to-maturity securities |  |
Amortized cost | 2,575 | 2,450
Estimated fair value | 2,575 | 2,450
Carrying value | 2,575 | 2,450
U.S. Treasury securities |  |
Available-for-sale securities |  |
Amortized cost |  | 1,327
Estimated fair value |  | 1,327
Carrying value |  | 1,327
Residential mortgage-backed securities |  |
Available-for-sale securities |  |
Amortized cost | 4,616,720 | 4,876,573
Gross unrealized gains | 144,020 | 77,741
Gross unrealized Losses | (11,159) | (29,846)
Estimated fair value | 4,749,581 | 4,924,468
Carrying value | 4,749,581 | 4,924,468
Mortgage-backed securities by Ginnie Mae |  |
Available-for-sale securities |  |
Carrying value | 2,958,000 | 2,326,378
Mortgage-backed securities by Fannie Mae and Freddie Mac |  |
Available-for-sale securities |  |
Carrying value | 1,751,104 | 2,552,062
Mortgage-backed securities by non-government entities |  |
Available-for-sale securities |  |
Carrying value | 40,477 | 46,028
Obligations of states and political subdivisions |  |
Available-for-sale securities |  |
Amortized cost | 210,178 | 150,122
Gross unrealized gains | 15,257 | 636
Gross unrealized Losses | (920) | (4,761)
Estimated fair value | 224,515 | 145,997
Carrying value | 224,515 | 145,997
Equity Securities |  |
Available-for-sale securities |  |
Amortized cost | 13,825 | 13,825
Gross unrealized gains | 1,031 | 864
Gross unrealized Losses | (39) | (24)
Estimated fair value | 14,817 | 14,665
Carrying value | $ 14,817 | $ 14,665

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