Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost, Net of Allowance for Credit Losses	$ 481.3
Due after one year through five years, Amortized Cost, Net of Allowance for Credit Losses	2,744.1
Due after five years through ten years, Amortized Cost, Net of Allowance for Credit Losses	2,403.5
Due after ten years, Amortized Cost, Net of Allowance for Credit Losses	302.9
Gross fixed maturities, Amortized Cost, Net of Allowance for Credit Losses	5,931.8
Mortgage-backed and other asset-backed securities, Amortized Cost, Net of Allowance for Credit Losses	2,713.6
Amortized Cost, Net of Allowance for Credit Losses	8,645.4
Due in one year or less, Fair Value	476.7
Due after one year through five years, Fair Value	2,641.8
Due after five years through ten years, Fair Value	2,136.9
Due after ten years, Fair Value	271.2
Gross fixed maturities, Fair Value	5,526.6
Mortgage-backed and other asset-backed securities, Fair Value	2,488.8
Fixed maturities, Fair Value	$ 8,015.4	$ 7,985.3