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Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 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 | 5,470,103 | 5,423,189
Gross unrealized gains | 81,235 | 119,670
Gross unrealized losses | (102,588) | (17,844)
Total investment securities | 5,448,750 | 5,525,015
Estimated fair value |  |
Available-for-sale securities |  |
Total investment securities | 5,448,750 | 5,525,015
Carrying value |  |
Available-for-sale securities |  |
Total investment securities | 5,448,750 | 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 | 5,197,331 | 5,186,652
Gross unrealized gains | 70,436 | 94,585
Gross unrealized losses | (95,581) | (16,033)
Residential mortgage-backed securities | Estimated fair value |  |
Available-for-sale securities |  |
Total investment securities | 5,172,186 | 5,265,204
Residential mortgage-backed securities | Carrying value |  |
Available-for-sale securities |  |
Total investment securities | 5,172,186 | 5,265,204
Mortgage-backed securities by Ginnie Mae |  |
Available-for-sale securities |  |
Total investment securities | 1,894,858 | 2,035,742
Mortgage-backed securities by Fannie Mae and Freddie Mac |  |
Available-for-sale securities |  |
Total investment securities | 3,248,329 | 3,196,602
Mortgage-backed securities by non-government entities |  |
Available-for-sale securities |  |
Total investment securities | 28,999 | 32,860
Obligations of states and political subdivisions |  |
Available-for-sale securities |  |
Amortized cost | 244,697 | 216,962
Gross unrealized gains | 9,758 | 23,504
Gross unrealized losses | (6,724) | (1,791)
Obligations of states and political subdivisions | Estimated fair value |  |
Available-for-sale securities |  |
Total investment securities | 247,731 | 238,675
Obligations of states and political subdivisions | Carrying value |  |
Available-for-sale securities |  |
Total investment securities | 247,731 | 238,675
Equity Securities |  |
Available-for-sale securities |  |
Amortized cost | 28,075 | 19,575
Gross unrealized gains | 1,041 | 1,581
Gross unrealized losses | (283) | (20)
Equity Securities | Estimated fair value |  |
Available-for-sale securities |  |
Total investment securities | 28,833 | 21,136
Equity Securities | Carrying value |  |
Available-for-sale securities |  |
Total investment securities | $ 28,833 | $ 21,136

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