Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 34,443.6	$ 33,056.6
Gross Unrealized Gain	613.4	822.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,665.3	$ 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	$ 33,159.5
Gross Unrealized Gain	592.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,610.6
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	$ 1,284.1
Gross Unrealized Gain	20.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 54.7
Fixed maturity securities in unrealized loss position percent of total	2.10%