Investments (Amortized costs and Estimated Fair Value) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in One Year or Less	$ 161.0
Due after One Year to Five Years	905.8
Due after Five Years to Ten Years	1,332.6
Due after Ten Years	1,952.8
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	707.8
Investments in Fixed Maturities	5,060.0	$ 5,021.6
Fair Value
Due in One Year or Less	161.6
Due after One Year to Five Years	918.9
Due after Five Years to Ten Years	1,376.0
Due after Ten Years	2,136.9
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	707.8
Investments in Fixed Maturities	$ 5,301.2	$ 5,382.7