Investments (Amortized costs and Estimated Fair Value) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in One Year or Less	$ 154.0
Due after One Year to Five Years	1,087.6
Due after Five Years to Ten Years	1,717.7
Due after Ten Years	1,884.4
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,128.7
Investments in Fixed Maturities	5,972.4	$ 5,021.6
Fair Value
Due in One Year or Less	156.0
Due after One Year to Five Years	1,094.9
Due after Five Years to Ten Years	1,735.8
Due after Ten Years	2,006.2
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,115.7
Investments in Fixed Maturities	$ 6,108.6	$ 5,382.7