Fair Value (Fair Value By Level) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Available for sale debt securities	$ 3,192,872	$ 3,411,350
Equity Securities	6,172	5,937
Marketable Securities	3,201,444	3,418,487
Assets/liabilities measured at fair value
Assets:
Available for sale debt securities	3,192,872
Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,097,410	3,223,010
Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	95,462	188,340
Measured on a recurring basis: | Assets/liabilities measured at fair value
Assets:
Marketable Securities	3,199,043	3,417,287
Measured on a recurring basis: | Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,097,409	3,223,010
Measured on a recurring basis: | Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	95,462	188,340
Measured on a recurring basis: | Assets/liabilities measured at fair value | Equity securities with readily determinable fair values
Assets:
Equity Securities	6,172	5,937
Measured on a recurring basis: | Assets/liabilities measured at fair value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable Securities	6,172	5,937
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	6,172	5,937
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2)
Assets:
Marketable Securities	3,192,871	3,411,350
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	3,097,409	3,223,010
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	$ 95,462	$ 188,340