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Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
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 | 5,102,700 | 5,423,189
Gross Unrealized Gains | 106,900 | 119,670
Gross Unrealized Losses | (29,326) | (17,844)
Estimated Fair Value | 5,180,274 | 5,525,015
Carrying Value | 5,180,274 | 5,525,015
Other securities |  |
Held-to-maturity securities |  |
Amortized Cost | 2,400 | 2,400
Estimated Fair Value | 2,400 | 2,400
Carrying Value | 2,400 | 2,400
Residential mortgage-backed securities |  |
Available-for-sale securities |  |
Amortized Cost | 4,865,035 | 5,186,652
Gross Unrealized Gains | 85,129 | 94,585
Gross Unrealized Losses | (26,953) | (16,033)
Estimated Fair Value | 4,923,211 | 5,265,204
Carrying Value | 4,923,211 | 5,265,204
Mortgage-backed securities by Ginnie Mae |  |
Available-for-sale securities |  |
Carrying Value | 1,893,174 | 2,035,742
Mortgage-backed securities by Fannie Mae and Freddie Mac |  |
Available-for-sale securities |  |
Carrying Value | 2,999,340 | 3,196,602
Mortgage-backed securities by non-government entities |  |
Available-for-sale securities |  |
Carrying Value | 30,697 | 32,860
Obligations of states and political subdivisions |  |
Available-for-sale securities |  |
Amortized Cost | 218,090 | 216,962
Gross Unrealized Gains | 20,162 | 23,504
Gross Unrealized Losses | (2,342) | (1,791)
Estimated Fair Value | 235,910 | 238,675
Carrying Value | 235,910 | 238,675
Equity Securities |  |
Available-for-sale securities |  |
Amortized Cost | 19,575 | 19,575
Gross Unrealized Gains | 1,609 | 1,581
Gross Unrealized Losses | (31) | (20)
Estimated Fair Value | 21,153 | 21,136
Carrying Value | $ 21,153 | $ 21,136

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