Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,216.3
Due after one year through five years	10,723.2
Due after five years through ten years	14,142.4
Due after ten years	25,314.6
Subtotal	52,396.5
Mortgage-backed and other asset-backed securities	18,414.6
Amortized cost, fixed maturities	70,811.1	$ 70,546.8
Fair value of fixed maturities available-for-sale
Due in one year or less	2,245.6
Due after one year through five years	11,323.9
Due after five years through ten years	15,275.6
Due after ten years	28,899.4
Subtotal	57,744.5
Mortgage-backed and other asset-backed securities	18,812.6
Fair value, fixed maturities	$ 76,557.1	$ 78,710.3