Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,652.6
Due after one year through five years	11,417.8
Due after five years through ten years	10,047.6
Due after ten years	19,136.1
Subtotal	43,254.1
Mortgage-backed and other asset-backed securities	13,308.0
Total	56,562.1
Fair value of fixed maturities available-for-sale
Due in one year or less	2,669.2
Due after one year through five years	11,728.4
Due after five years through ten years	10,399.7
Due after ten years	21,290.6
Subtotal	46,087.9
Mortgage-backed and other asset-backed securities	13,300.5
Total	$ 59,388.4	$ 54,846.1