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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Assets:    
Available for sale securities $ 5,189,977 $ 5,525,015
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value
   
Assets:    
Available for sale securities 5,189,977 5,525,015
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
   
Assets:    
Available for sale securities 4,920,978 5,265,204
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | States and political subdivisions
   
Assets:    
Available for sale securities 248,626 238,675
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Other
   
Assets:    
Available for sale securities 20,373 21,136
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1)
   
Assets:    
Available for sale securities 20,373 21,136
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
   
Assets:    
Available for sale securities 20,373 21,136
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2)
   
Assets:    
Available for sale securities 5,139,163 5,471,019
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
   
Assets:    
Available for sale securities 4,890,537 5,232,344
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | States and political subdivisions
   
Assets:    
Available for sale securities 248,626 238,675
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3)
   
Assets:    
Available for sale securities 30,441 32,860
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
   
Assets:    
Available for sale securities $ 30,441 $ 32,860