Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 36,247.8	$ 36,833.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	902.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,037.4
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	$ 34,759.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	886.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,943.1
Fixed maturity securities in unrealized loss position percent of total	96.90%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,487.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	16.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 94.3
Fixed maturity securities in unrealized loss position percent of total	3.10%