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Fair Value (Fair Value By Level) (Details) - USD ($) | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Assets: |  |
Available for sale debt securities | $ 3,720,566,000 | $ 4,124,185,000
Equity Securities | 27,054,000 | 27,885,000
Marketable Securities | 3,750,020,000 | 4,154,470,000
Assets/liabilities measured at fair value |  |
Assets: |  |
Available for sale debt securities | 3,720,566,000 |
Assets/liabilities measured at fair value | Residential mortgage-backed securities |  |
Assets: |  |
Available for sale debt securities | 3,509,643,000 | 3,891,233,000
Assets/liabilities measured at fair value | Obligations of states and political subdivisions |  |
Assets: |  |
Available for sale debt securities | 210,923,000 | 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 | 3,747,620,000 | 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 | 3,509,643,000 | 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 | 210,923,000 | 232,951,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Equity securities |  |
Assets: |  |
Equity Securities | 27,054,000 | 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 | 27,054,000 | 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 | 27,054,000 | 27,886,000
Measured on a recurring basis: | Assets/liabilities measured at fair value | Significant Other Observable Inputs (Level 2) |  |
Assets: |  |
Marketable Securities | 3,720,566,000 | 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 | 3,509,643,000 | 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 | $ 210,923,000 | $ 232,951,000

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