Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 34,070.7	$ 34,840.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	424.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 4,578.3
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	$ 32,659.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	420.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 4,404.0
Fixed maturity securities in unrealized loss position percent of total	96.20%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,411.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 174.3
Fixed maturity securities in unrealized loss position percent of total	3.80%