Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,509.4
Due after one year through five years	11,198.7
Due after five years through ten years	10,438.7
Due after ten years	19,874.7
Subtotal	44,021.5
Mortgage-backed and other asset-backed securities	14,483.6
Total	58,505.1
Fair value of fixed maturities available-for-sale
Due in one year or less	2,512.6
Due after one year through five years	11,279.3
Due after five years through ten years	10,387.0
Due after ten years	20,855.9
Subtotal	45,034.8
Mortgage-backed and other asset-backed securities	14,276.9
Total	$ 59,311.7	$ 59,388.4