Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,747.4
Due after one year through five years	10,602.1
Due after five years through ten years	10,511.7
Due after ten years	19,673.4
Subtotal	43,534.6
Mortgage-backed and other asset-backed securities	16,191.2
Amortized cost, fixed maturities	59,725.8	$ 56,562.1
Fair value of fixed maturities available-for-sale
Due in one year or less	2,753.5
Due after one year through five years	10,631.7
Due after five years through ten years	10,389.7
Due after ten years	20,321.6
Subtotal	44,096.5
Mortgage-backed and other asset-backed securities	16,012.0
Fair value, fixed maturities	$ 60,108.5	$ 59,388.4