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Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Held-to-maturity securities |  |
Amortized Cost | $ 2,400 | $ 2,450
Estimated Fair Value | 2,400 | 2,450
Carrying Value | 2,400 | 2,450
Available-for-sale securities |  |
Amortized Cost | 5,423,189 | 5,082,095
Gross Unrealized Gains | 119,670 | 143,760
Gross Unrealized Losses | (17,844) | (11,940)
Estimated Fair Value | 5,525,015 | 5,213,915
Carrying Value | 5,525,015 | 5,213,915
Other securities |  |
Held-to-maturity securities |  |
Amortized Cost | 2,400 | 2,450
Estimated Fair Value | 2,400 | 2,450
Carrying Value | 2,400 | 2,450
Residential mortgage-backed securities |  |
Available-for-sale securities |  |
Amortized Cost | 5,186,652 | 4,851,747
Gross Unrealized Gains | 94,585 | 128,196
Gross Unrealized Losses | (16,033) | (10,680)
Estimated Fair Value | 5,265,204 | 4,969,263
Carrying Value | 5,265,204 | 4,969,263
Mortgage-backed securities by Ginnie Mae |  |
Available-for-sale securities |  |
Carrying Value | 2,035,742 | 3,008,935
Mortgage-backed securities by Fannie Mae and Freddie Mac |  |
Available-for-sale securities |  |
Carrying Value | 3,196,602 | 1,920,723
Mortgage-backed securities by non-government entities |  |
Available-for-sale securities |  |
Carrying Value | 32,860 | 39,605
Obligations of states and political subdivisions |  |
Available-for-sale securities |  |
Amortized Cost | 216,962 | 211,523
Gross Unrealized Gains | 23,504 | 14,449
Gross Unrealized Losses | (1,791) | (1,211)
Estimated Fair Value | 238,675 | 224,761
Carrying Value | 238,675 | 224,761
Equity Securities |  |
Available-for-sale securities |  |
Amortized Cost | 19,575 | 18,825
Gross Unrealized Gains | 1,581 | 1,115
Gross Unrealized Losses | (20) | (49)
Estimated Fair Value | 21,136 | 19,891
Carrying Value | $ 21,136 | $ 19,891

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