Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 36,833.9	$ 34,840.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,189.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,763.9
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	$ 35,344.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,169.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,660.3
Fixed maturity securities in unrealized loss position percent of total	96.30%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,489.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 103.6
Fixed maturity securities in unrealized loss position percent of total	3.70%