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Fair Value (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010
Assets: |  |
Available-for-sale | $ 5,213,915,000 | $ 5,086,457,000
Non-financial assets: |  |
Other real estate owned | 86,626,000 | 87,192,000
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Charges to allowance for probable loan losses in connection with other real estate owned | 1,100,000 | 22,790,000
Write downs in fair value in connection with other real estate owned | 9,509,000 | 719,000
Significant Unobservable Inputs (Level 3) |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, discount rate (as a percent) | 13.00% |
Default rate assumptions, thereafter (as a percent) | 7.00% |
Loss severity rate assumptions, first year (as a percent) | 60.00% |
Decrease in loss severity rates, following three years (as a percent) | 10.00% |
Decrease in loss severity rates, thereafter (as a percent) | 20.00% |
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | U.S. Treasury securities |  |
Assets: |  |
Available-for-sale |  | 1,327,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities |  |
Assets: |  |
Available-for-sale | 4,969,263,000 | 4,924,468,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | States and political subdivisions |  |
Assets: |  |
Available-for-sale | 224,761,000 | 145,997,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Other |  |
Assets: |  |
Available-for-sale | 19,891,000 | 14,665,000
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other |  |
Assets: |  |
Available-for-sale | 19,891,000 | 14,665,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities |  |
Assets: |  |
Available-for-sale |  | 1,327,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities |  |
Assets: |  |
Available-for-sale | 4,929,658,000 | 4,878,440,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | States and political subdivisions |  |
Assets: |  |
Available-for-sale | 224,761,000 | 145,997,000
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities |  |
Assets: |  |
Available-for-sale | 39,605,000 | 46,028,000
Measured on a non-recurring basis: |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Change in provision, impaired loans | 15,457,000 | (114,000)
Change in provision, other real estate owned | 9,509,000 | 719,000
Measured on a non-recurring basis: | Assets/Liabilities Measured at Fair Value |  |
Assets: |  |
Impaired Loans | 81,723,000 | 24,034,000
Non-financial assets: |  |
Other real estate owned | 34,631,000 | 4,304,000
Measured on a non-recurring basis: | Significant Unobservable Inputs (Level 3) |  |
Assets: |  |
Impaired Loans | 81,723,000 | 24,034,000
Non-financial assets: |  |
Other real estate owned | $ 34,631,000 | $ 4,304,000
Maximum |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 11.00% |
Default rate assumptions for the first year, low end of range (as a percent) | 60.00% |
Maximum | Significant Unobservable Inputs (Level 3) |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 5.00% |
Default rate assumptions for the first year, low end of range (as a percent) | 11.00% |
Minimum |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 10.00% |
Default rate assumptions for the first year, low end of range (as a percent) | 7.00% |
Minimum | Significant Unobservable Inputs (Level 3) |  |
Assumptions used in discounted cash flow model to determine fair value of investments classified within level 3 |  |
Estimated future principal prepayment rate assumption, low end of range (as a percent) | 4.00% |
Default rate assumptions for the first year, low end of range (as a percent) | 10.00% |

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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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