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Investment Securities (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | ---
Held-to-maturity securities |  |
Amortized cost | $ 2,450 | $ 2,450
Estimated fair value | 2,450 | 2,450
Carrying value | 2,450 | 2,450
Available-for-sale securities |  |
Amortized cost | 5,082,095 | 5,041,847
Gross unrealized gains | 143,760 | 79,241
Gross unrealized Losses | (11,940) | (34,631)
Estimated fair value | 5,213,915 | 5,086,457
Carrying value | 5,213,915 | 5,086,457
Other securities |  |
Held-to-maturity securities |  |
Amortized cost | 2,450 | 2,450
Estimated fair value | 2,450 | 2,450
Carrying value | 2,450 | 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 | 4,851,747 | 4,876,573
Gross unrealized gains | 128,196 | 77,741
Gross unrealized Losses | (10,680) | (29,846)
Estimated fair value | 4,969,263 | 4,924,468
Carrying value | 4,969,263 | 4,924,468
Mortgage-backed securities by Ginnie Mae |  |
Available-for-sale securities |  |
Carrying value | 3,008,935 | 2,326,378
Mortgage-backed securities by Fannie Mae and Freddie Mac |  |
Available-for-sale securities |  |
Carrying value | 1,920,723 | 2,552,062
Mortgage-backed securities by non-government entities |  |
Available-for-sale securities |  |
Carrying value | 39,605 | 46,028
Obligations of states and political subdivisions |  |
Available-for-sale securities |  |
Amortized cost | 211,523 | 150,122
Gross unrealized gains | 14,449 | 636
Gross unrealized Losses | (1,211) | (4,761)
Estimated fair value | 224,761 | 145,997
Carrying value | 224,761 | 145,997
Equity Securities |  |
Available-for-sale securities |  |
Amortized cost | 18,825 | 13,825
Gross unrealized gains | 1,115 | 864
Gross unrealized Losses | (49) | (24)
Estimated fair value | 19,891 | 14,665
Carrying value | $ 19,891 | $ 14,665

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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
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