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Investments (Amortized costs and Estimated Fair Value) (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Amortized Cost |  |
Due in One Year or Less | $ 147.4 |
Due after One Year to Five Years | 928.2 |
Due after Five Years to Ten Years | 1,380.1 |
Due after Ten Years | 1,907.7 |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 712.6 |
Investments in Fixed Maturities | 5,076.0 | $ 5,021.6
Fair Value |  |
Due in One Year or Less | 149.7 |
Due after One Year to Five Years | 935.6 |
Due after Five Years to Ten Years | 1,408.3 |
Due after Ten Years | 2,060.1 |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 706.6 |
Investments in Fixed Maturities | $ 5,260.3 | $ 5,382.7

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