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Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
Due in One Year or Less, Amortized Cost | $ 130.3 |
Due in One Year or Less, Fair Value | 131.0 |
Due after One Year to Five Years, Amortized Cost | 914.3 |
Due after One Year to Five Years, Fair Value | 917.7 |
Due after Five Years to Ten Years, Amortized Cost | 1,816.8 |
Due after Five Years to Ten Years, Fair Value | 1,842.5 |
Due after Ten Years, Amortized Cost | 2,234.8 |
Due after Ten Years, Fair Value | 2,358.9 |
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost | 1,188.3 |
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value | 1,174.1 |
Amortized Cost | 6,284.5 | $ 5,021.6
Investments in Fixed Maturities, Fair Value | 6,424.2 | $ 5,382.7
Government National Mortgage Association Certificates and Obligations (GNMA) [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value | 618.5 |
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value | 8.4 |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value | 7.6 |
Other Non-Governmental Issuers [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value | $ 539.6 |

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