Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,149.5
Due after one year through five years	10,909.1
Due after five years through ten years	13,574.9
Due after ten years	24,151.9
Subtotal	50,785.4
Mortgage-backed and other asset-backed securities	18,568.5
Amortized cost, fixed maturities	69,353.9	$ 65,302.5
Fair value of fixed maturities available-for-sale
Due in one year or less	2,175.2
Due after one year through five years	11,539.8
Due after five years through ten years	14,826.5
Due after ten years	28,916.0
Subtotal	57,457.5
Mortgage-backed and other asset-backed securities	19,036.6
Fair value, fixed maturities	$ 76,494.1	$ 70,106.2