Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 35,629.9	$ 36,833.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	676.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,314.0
Fixed maturity securities in unrealized loss position percent of total	100.00%
External Credit Rating, Investment Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 34,193.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	661.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,236.2
Fixed maturity securities in unrealized loss position percent of total	97.70%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,436.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	15.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 77.8
Fixed maturity securities in unrealized loss position percent of total	2.30%