Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Amortized Cost
Due in One Year or Less	$ 259.7
Due after One Year to Five Years	893.5
Due after Five Years to Ten Years	981.3
Due after Ten Years	3,524.7
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,825.1
Fixed maturities, at amortized cost	7,484.3	$ 7,295.0
Fair Value
Due in One Year or Less	255.7
Due after One Year to Five Years	868.8
Due after Five Years to Ten Years	885.7
Due after Ten Years	2,946.2
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,712.7
Fair Value	$ 6,669.1	$ 6,409.6