Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Amortized Cost
Due in One Year or Less	$ 333.0
Due after One Year to Five Years	873.4
Due after Five Years to Ten Years	1,003.8
Due after Ten Years	3,474.0
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,688.0
Fixed maturities, at amortized cost	7,372.2	$ 7,295.0
Fair Value
Due in One Year or Less	327.5
Due after One Year to Five Years	848.1
Due after Five Years to Ten Years	893.9
Due after Ten Years	2,915.8
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,573.3
Fair Value	$ 6,558.6	$ 6,409.6