Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,674.0
Due after one year through five years	13,074.3
Due after five years through ten years	8,333.4
Due after ten years	12,823.8
Subtotal	36,905.5
Mortgage-backed and other asset-backed securities	11,768.8
Total	48,674.3
Fair value of fixed maturities available-for-sale
Due in one year or less	2,695.0
Due after one year through five years	13,379.1
Due after five years through ten years	8,460.7
Due after ten years	13,619.3
Subtotal	38,154.1
Mortgage-backed and other asset-backed securities	11,812.4
Total	$ 49,966.5	$ 49,670.8