Investments - Amortized Costs and Estimated Fair Value (Details) $ in Millions	Sep. 30, 2024 USD ($)
Amortized Cost
Due in One Year or Less	$ 264.2
Due after One Year to Five Years	874.3
Due after Five Years to Ten Years	1,031.0
Due after Ten Years	3,561.0
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,728.9
Fixed maturities, at amortized cost	7,459.4
Fair Value
Due in One Year or Less	261.2
Due after One Year to Five Years	856.2
Due after Five Years to Ten Years	929.6
Due after Ten Years	3,200.6
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,624.4
Fair Value	$ 6,872.0