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Investments - Available-for-sale Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Investments Debt And Equity Securities [Abstract] |  |
Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis | $ 1,193.2 | $ 529.2

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- Definition Available-for-sale fixed-maturities with cost basis in excess of fair values, cost basis. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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