Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Amortized Cost
Due in One Year or Less	$ 337.0
Due after One Year to Five Years	773.5
Due after Five Years to Ten Years	1,021.6
Due after Ten Years	3,496.9
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,666.0
Amortized Cost	7,295.0	$ 7,565.8
Fair Value
Due in One Year or Less	332.3
Due after One Year to Five Years	746.3
Due after Five Years to Ten Years	890.0
Due after Ten Years	2,903.7
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,537.3
Investments in Fixed Maturities	$ 6,409.6	$ 6,881.9