Investments (Mortgage Loans and Valuation Allowance by Portfolio Segment) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	$ 704		$ 733
Evaluated collectively for credit losses	54,822		53,540
Total mortgage loans	55,526		54,273
Specific credit losses	76		117
Non-specifically identified credit losses	250		230
Valuation allowances	326	302	347	354	361	481
Mortgage loan, net of valuation allowance	55,200		53,926
Commercial mortgage loans portfolio segment [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	520		539
Evaluated collectively for credit losses	39,742		39,933
Total mortgage loans	40,262		40,472
Specific credit losses	62		94
Non-specifically identified credit losses	197		199
Valuation allowances	259	242	293	300	300	398
Mortgage loan, net of valuation allowance	40,003		40,179
Agricultural [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	170		181
Evaluated collectively for credit losses	12,660		12,662
Total mortgage loans	12,830		12,843
Specific credit losses	12		21
Non-specifically identified credit losses	38		31
Valuation allowances	50	49	52	52	59	81
Mortgage loan, net of valuation allowance	12,780		12,791
Residential [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	14		13
Evaluated collectively for credit losses	2,420		945
Total mortgage loans	2,434		958
Specific credit losses	2		2
Non-specifically identified credit losses	15		0
Valuation allowances	17	11	2	2	2	2
Mortgage loan, net of valuation allowance	$ 2,417		$ 956