Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 35,966.0	$ 35,629.9
Gross Unrealized Gain	853.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,856.6
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	$ 34,513.8
Gross Unrealized Gain	827.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,797.8
Fixed maturity securities in unrealized loss position percent of total	97.90%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,452.2
Gross Unrealized Gain	25.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 58.8
Fixed maturity securities in unrealized loss position percent of total	2.10%