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Fair Value Measurements (Details) (USD $)
Mar. 31, 2014
Dec. 31, 2013
Assets:    
Available for sale securities $ 5,222,557,000 $ 5,304,579,000
Assets/Liabilities Measured at Fair Value
    
Assets:    
Available for sale securities 5,222,557,000  
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value
    
Assets:    
Available for sale securities 5,222,557,000 5,304,579,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
    
Assets:    
Available for sale securities 4,932,505,000 5,027,701,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | States and political subdivisions
    
Assets:    
Available for sale securities 261,125,000 248,410,000
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Other
    
Assets:    
Available for sale securities 28,927,000 28,468,000
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1)
    
Assets:    
Available for sale securities 28,927,000 28,468,000
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
    
Assets:    
Available for sale securities 28,927,000 28,468,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2)
    
Assets:    
Available for sale securities 5,166,674,000 5,248,259,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
    
Assets:    
Available for sale securities 4,905,549,000 4,999,849,000
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | States and political subdivisions
    
Assets:    
Available for sale securities 261,125,000 248,410,000
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3)
    
Assets:    
Available for sale securities 26,956,000 27,852,000
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
    
Assets:    
Available for sale securities $ 26,956,000 $ 27,852,000