Investments (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 3,829.6
Due after one year through five years	12,706.2
Due after five years through ten years	9,289.4
Due after ten years	11,296.0
Subtotal	37,121.2
Mortgage-backed and other asset-backed securities	11,336.0
Total	48,457.2
Fair value of fixed maturities available-for-sale
Due in one year or less	3,892.5
Due after one year through five years	13,387.2
Due after five years through ten years	10,410.0
Due after ten years	12,610.0
Subtotal	40,299.7
Mortgage-backed and other asset-backed securities	11,234.2
Total	$ 51,533.9	$ 49,006.7