Investments (Schedule of mortgage loan on real estate credit quality indicator) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Mortgage Loans Credit Quality [Line Items]
Principal amount of mortgage loans on real estate	$ 7,043	$ 6,960
Percentage of total mortgage loans on real estate	100.00%	100.00%
Loan to value ratio less than sixty five [Member]
Mortgage Loans Credit Quality [Line Items]
Principal amount of mortgage loans on real estate	5,677	5,338
Percentage of total mortgage loans on real estate	80.60%	76.70%
Debt-service coverage ratio	1.68	1.61
Loan to value ratio sixty five to seventy four [Member]
Mortgage Loans Credit Quality [Line Items]
Principal amount of mortgage loans on real estate	897	1,198
Percentage of total mortgage loans on real estate	12.70%	17.20%
Debt-service coverage ratio	1.39	1.37
Loan to value ratio seventy five to one hundred [Member]
Mortgage Loans Credit Quality [Line Items]
Principal amount of mortgage loans on real estate	386	308
Percentage of total mortgage loans on real estate	5.50%	4.40%
Debt-service coverage ratio	0.84	0.92
Loan to value ratio, greater than one hundred [Member]
Mortgage Loans Credit Quality [Line Items]
Principal amount of mortgage loans on real estate	$ 83	$ 116
Percentage of total mortgage loans on real estate	1.20%	1.70%
Debt-service coverage ratio	0.66	0.36