Investments (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 3,605.4
Due after one year through five years	12,713.2
Due after five years through ten years	9,131.5
Due after ten years	10,890.0
Subtotal	36,340.1
Mortgage-backed and other asset-backed securities	11,187.7
Total	47,527.8
Fair value of fixed maturities available-for-sale
Due in one year or less	3,657.7
Due after one year through five years	13,231.7
Due after five years through ten years	10,058.0
Due after ten years	11,956.8
Subtotal	38,904.2
Mortgage-backed and other asset-backed securities	10,889.4
Total	$ 49,793.6	$ 49,006.7