Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 35,374.1	$ 34,840.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	810.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,464.6
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	$ 33,559.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	801.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 3,314.9
Fixed maturity securities in unrealized loss position percent of total	95.70%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,815.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 149.7
Fixed maturity securities in unrealized loss position percent of total	4.30%