Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 32,994.5	$ 33,056.6
Gross Unrealized Gain	227.9	822.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,451.6	$ 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	$ 31,800.7
Gross Unrealized Gain	221.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,376.5
Fixed maturity securities in unrealized loss position percent of total	97.80%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,193.8
Gross Unrealized Gain	6.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 75.1
Fixed maturity securities in unrealized loss position percent of total	2.20%