Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 44,137.3	$ 47,443.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,653.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 55.9
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	$ 40,637.2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,371.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 18.1
Fixed maturity securities in unrealized loss position percent of total	32.40%
External Credit Rating, Below-Investment-Grade
Debt Securities, Available-for-sale
Fixed Maturity Securities	$ 3,500.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	282.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 37.8
Fixed maturity securities in unrealized loss position percent of total	67.60%