Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 36,160.2	$ 34,840.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,085.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,026.6
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	$ 34,194.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,070.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,876.7
Fixed maturity securities in unrealized loss position percent of total	95.00%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,966.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 149.9
Fixed maturity securities in unrealized loss position percent of total	5.00%