Fair Value Measurements (Fair Value By Level) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Available for sale debt securities	$ 3,594,514	$ 3,378,923
Equity Securities	6,125	6,095
Marketable Securities	3,603,039	3,387,418
Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,520,332	3,285,548
Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	74,182	93,375
Measured on a recurring basis: | Assets/liabilities measured at fair value
Assets:
Marketable Securities	3,600,639	3,385,018
Measured on a recurring basis: | Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,520,332	3,285,548
Measured on a recurring basis: | Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	74,182	93,375
Measured on a recurring basis: | Assets/liabilities measured at fair value | Equity securities with readily determinable fair values
Assets:
Equity Securities	6,125	6,095
Measured on a recurring basis: | Assets/liabilities measured at fair value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable Securities	6,125	6,095
Measured on a recurring basis: | Assets/liabilities measured at fair value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities with readily determinable fair values
Assets:
Equity Securities	6,125	6,095
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2)
Assets:
Marketable Securities	3,594,514	3,378,923
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2) | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,520,332	3,285,548
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2) | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	$ 74,182	$ 93,375