Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in One Year or Less	$ 74.2
Due after One Year to Five Years	1,057.5
Due after Five Years to Ten Years	1,608.3
Due after Ten Years	3,002.9
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,349.7
Fixed maturities, at amortized cost	7,092.6	$ 6,692.7
Fair Value
Due in One Year or Less	77.7
Due after One Year to Five Years	1,121.4
Due after Five Years to Ten Years	1,748.1
Due after Ten Years	3,505.3
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,382.5
Fixed Maturities at Fair Value (Amortized Cost: 2021 - $7,092.6; 2020 - $6,692.7 Allowance for Credit Losses: 2021 - $5.1; 2020 - $3.3)	$ 7,835.0	$ 7,605.9