Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 33,300.8	$ 33,056.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	620.8	822.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,850.8	$ 2,506.7
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	$ 32,046.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	605.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,784.0
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	$ 1,254.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	15.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 66.8
Fixed maturity securities in unrealized loss position percent of total	2.30%