Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Due in One Year or Less	$ 81.4
Due after One Year to Five Years	867.8
Due after Five Years to Ten Years	1,641.3
Due after Ten Years	2,538.4
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,170.8
Investments in Fixed Maturities	6,299.7	$ 6,284.5
Fair Value
Due in One Year or Less	81.8
Due after One Year to Five Years	890.1
Due after Five Years to Ten Years	1,767.9
Due after Ten Years	2,954.2
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,189.6
Investments in Fixed Maturities	$ 6,883.6	$ 6,424.2