Investments (Details 35) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	$ 161	$ 217
Interest Accrual Status of Mortgage Loans	31	29
Mortgage loans that were in non accrual status	161	237
Commercial mortgage loans portfolio segment [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	0	63
Interest Accrual Status of Mortgage Loans	0	0
Mortgage loans that were in non accrual status	0	63
Agricultural mortgage loans portfolio segment [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	151	146
Interest Accrual Status of Mortgage Loans	31	29
Mortgage loans that were in non accrual status	143	157
Residential mortgage loans portfolio segment [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	10	8
Interest Accrual Status of Mortgage Loans	0	0
Mortgage loans that were in non accrual status	$ 18	$ 17