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