Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Amortized Cost
Due in One Year or Less	$ 173.8
Due after One Year to Five Years	903.0
Due after Five Years to Ten Years	1,069.5
Due after Ten Years	3,615.5
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,804.0
Amortized Cost	7,565.8	$ 7,811.8
Fair Value
Due in One Year or Less	169.9
Due after One Year to Five Years	876.3
Due after Five Years to Ten Years	940.1
Due after Ten Years	3,235.4
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,660.2
Investments in Fixed Maturities	$ 6,881.9	$ 6,894.8