Fair Value (Fair Value By Level) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Assets:
Available for sale debt securities	$ 3,868,173,000	$ 4,124,185,000
Equity securities	27,301,000	27,885,000
Marketable Securities	3,897,874,000	4,154,470,000
Assets/liabilities measured at fair value
Assets:
Available for sale debt securities	3,868,173,000
Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,656,083,000	3,891,233,000
Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	212,090,000	232,951,000
Assets/liabilities measured at fair value | Equity securities
Assets:
Equity securities		27,886,000
Measured on a recurring basis: | Assets/liabilities measured at fair value
Assets:
Marketable Securities	3,895,474,000	4,152,070,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Residential mortgage-backed securities
Assets:
Available for sale debt securities	3,656,083,000	3,891,233,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Obligations of states and political subdivisions
Assets:
Available for sale debt securities	212,090,000	232,951,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Equity securities
Assets:
Equity securities	27,301,000	27,886,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Marketable Securities	27,301,000	27,886,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets:
Equity securities	27,301,000	27,886,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2)
Assets:
Marketable Securities	3,868,173,000	4,124,184,000
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	3,656,083,000	3,891,233,000
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	$ 212,090,000	$ 232,951,000