Investments (Past Due and Interest Accrual Status of Mortgage Loans) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	$ 132	$ 147
Interest Accrual Status of Mortgage Loans	44	53
Mortgage loans that were in non accrual status	316	169
Commercial mortgage loans portfolio segment [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	0	2
Interest Accrual Status of Mortgage Loans	0	0
Mortgage loans that were in non accrual status	223	84
Agricultural [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	105	116
Interest Accrual Status of Mortgage Loans	44	53
Mortgage loans that were in non accrual status	75	67
Residential [Member]
Past Due and Interest Accrual Status of Mortgage Loans
Past Due Status of Mortgage Loans	27	29
Interest Accrual Status of Mortgage Loans	0	0
Mortgage loans that were in non accrual status	$ 18	$ 18