Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in One Year or Less	$ 122.6
Due after One Year to Five Years	1,060.5
Due after Five Years to Ten Years	1,616.8
Due after Ten Years	2,475.7
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,419.2
Investments in Fixed Maturities	6,694.8	$ 6,372.7
Fair Value
Due in One Year or Less	124.2
Due after One Year to Five Years	1,117.0
Due after Five Years to Ten Years	1,812.8
Due after Ten Years	3,012.0
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,438.8
Investments in Fixed Maturities	$ 7,504.8	$ 6,922.1