Investments (Mortgage Loans and Valuation Allowance by Portfolio Segment) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	$ 703		$ 733
Evaluated collectively for credit losses	52,817		53,540
Total mortgage loans	53,520		54,273
Specific credit losses	72		117
Non-specifically identified credit losses	230		230
Valuation allowances	302	332	347	361	446	481
Mortgage loan, net of valuation allowance	53,218		53,926
Commercial mortgage loans portfolio segment [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	530		539
Evaluated collectively for credit losses	38,580		39,933
Total mortgage loans	39,110		40,472
Specific credit losses	56		94
Non-specifically identified credit losses	186		199
Valuation allowances	242	275	293	300	368	398
Mortgage loan, net of valuation allowance	38,868		40,179
Agricultural [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	159		181
Evaluated collectively for credit losses	12,510		12,662
Total mortgage loans	12,669		12,843
Specific credit losses	14		21
Non-specifically identified credit losses	35		31
Valuation allowances	49	54	52	59	75	81
Mortgage loan, net of valuation allowance	12,620		12,791
Residential [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	14		13
Evaluated collectively for credit losses	1,727		945
Total mortgage loans	1,741		958
Specific credit losses	2		2
Non-specifically identified credit losses	9		0
Valuation allowances	11	3	2	2	3	2
Mortgage loan, net of valuation allowance	$ 1,730		$ 956