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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%