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Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 33,190.2	$ 35,629.9
Gross Unrealized Gain	857.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,513.3
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	$ 31,942.8
Gross Unrealized Gain	832.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,463.5
Fixed maturity securities in unrealized loss position percent of total	98.00%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,247.4
Gross Unrealized Gain	24.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 49.8
Fixed maturity securities in unrealized loss position percent of total	2.00%

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