Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 37,783.5	$ 36,833.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,338.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,374.4
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	$ 36,287.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,308.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 2,308.8
Fixed maturity securities in unrealized loss position percent of total	97.20%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 1,496.3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	30.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 65.6
Fixed maturity securities in unrealized loss position percent of total	2.80%