CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Investments
Amortized Cost of Fixed Maturity Securities	$ 38,385.4	$ 38,410.6
Debt Securities, Available-for-sale, Allowance for credit losses for Fixed Maturity Securities	2.2	2.2
Other Assets
Premium Receivable, Allowance for Credit Loss	30.0	29.5
Reinsurance Recoverable, Allowance for Credit Loss	$ 1.7	$ 1.7
Stockholders' Equity, Number of Shares, Par Value and Other Disclosure [Abstract]
Common Stock, Par Value (in dollars per share)	$ 0.10	$ 0.10
Common Stock, Shares Authorized	725,000,000	725,000,000
Common Stock, Shares Issued	195,208,726	194,588,625
Treasury Stock, Shares at Cost	3,755,025	1,216,528