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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Assets:    
Available for sale securities $ 5,448,750 $ 5,525,015
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value
    
Assets:    
Available for sale securities 5,448,750 5,525,015
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
    
Assets:    
Available for sale securities 5,172,186 5,265,204
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | States and political subdivisions
    
Assets:    
Available for sale securities 247,731 238,675
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Other
    
Assets:    
Available for sale securities 28,833 21,136
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1)
    
Assets:    
Available for sale securities 28,833 21,136
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
    
Assets:    
Available for sale securities 28,833 21,136
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2)
    
Assets:    
Available for sale securities 5,390,918 5,471,019
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
    
Assets:    
Available for sale securities 5,143,187 5,232,344
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | States and political subdivisions
    
Assets:    
Available for sale securities 247,731 238,675
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3)
    
Assets:    
Available for sale securities 28,999 32,860
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
    
Assets:    
Available for sale securities $ 28,999 $ 32,860