Fair Value Measurements (Estimated Cost and Fair Value) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Assets:
Estimated fair value, Available for sale securities	$ 4,337,940	$ 4,199,372
Assets/Liabilities Measured at Fair Value
Assets:
Estimated fair value, Available for sale securities	4,337,940	4,199,372
Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
Assets:
Estimated fair value, Available for sale securities	4,030,513	3,893,211
Assets/Liabilities Measured at Fair Value | States and political subdivisions
Assets:
Estimated fair value, Available for sale securities	278,556	277,704
Assets/Liabilities Measured at Fair Value | Other
Assets:
Estimated fair value, Available for sale securities	28,871	28,457
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value
Assets:
Estimated fair value, Available for sale securities	4,337,940	4,199,372
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
Assets:
Estimated fair value, Available for sale securities	4,030,513	3,893,210
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | States and political subdivisions
Assets:
Estimated fair value, Available for sale securities	278,556	277,705
Measured on a recurring basis: | Assets/Liabilities Measured at Fair Value | Other
Assets:
Estimated fair value, Available for sale securities	28,871	28,457
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Estimated fair value, Available for sale securities		28,457
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Estimated fair value, Available for sale securities		28,457
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Assets/Liabilities Measured at Fair Value
Assets:
Estimated fair value, Available for sale securities	28,871
Measured on a recurring basis: | Quoted Prices in Active Markets for Identical Assets (Level 1) | Assets/Liabilities Measured at Fair Value | Other
Assets:
Estimated fair value, Available for sale securities	28,871
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Assets/Liabilities Measured at Fair Value
Assets:
Estimated fair value, Available for sale securities	4,288,854	4,149,686
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
Assets:
Estimated fair value, Available for sale securities	4,010,298	3,871,981
Measured on a recurring basis: | Significant Other Observable Inputs (Level 2) | Assets/Liabilities Measured at Fair Value | States and political subdivisions
Assets:
Estimated fair value, Available for sale securities	278,556	277,705
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3)
Assets:
Estimated fair value, Available for sale securities		21,229
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities
Assets:
Estimated fair value, Available for sale securities		$ 21,229
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Assets/Liabilities Measured at Fair Value
Assets:
Estimated fair value, Available for sale securities	20,215
Measured on a recurring basis: | Significant Unobservable Inputs (Level 3) | Assets/Liabilities Measured at Fair Value | Residential mortgage-backed securities
Assets:
Estimated fair value, Available for sale securities	$ 20,215