Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 3,189.3
Due after one year through five years	11,984.6
Due after five years through ten years	9,530.0
Due after ten years	17,505.5
Subtotal	42,209.4
Mortgage-backed and other asset-backed securities	12,962.4
Total	55,171.8
Fair value of fixed maturities available-for-sale
Due in one year or less	3,219.9
Due after one year through five years	12,401.7
Due after five years through ten years	9,897.2
Due after ten years	19,284.9
Subtotal	44,803.7
Mortgage-backed and other asset-backed securities	12,996.1
Total	$ 57,799.8	$ 54,846.1