Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 35,952.8	$ 36,833.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	723.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,319.8
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	$ 34,515.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	709.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,227.0
Fixed maturity securities in unrealized loss position percent of total	97.20%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,437.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	13.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 92.8
Fixed maturity securities in unrealized loss position percent of total	2.80%