Investments (Details 31) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	$ 456		$ 268
Evaluated collectively for credit losses	54,903		53,990
Total mortgage loans	55,359		54,258
Specific credit losses	97		105
Non-specifically identified credit losses	257		376
Valuation allowances	354	361	481	529	566	664
Mortgage loan, net of valuation allowance	55,005		53,777
Commercial mortgage loans portfolio segment [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	336		96
Evaluated collectively for credit losses	41,605		40,344
Total mortgage loans	41,941		40,440
Specific credit losses	73		59
Non-specifically identified credit losses	227		339
Valuation allowances	300	300	398	428	469	562
Mortgage loan, net of valuation allowance	41,641		40,042
Agricultural mortgage loans portfolio segment [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	107		159
Evaluated collectively for credit losses	12,493		12,970
Total mortgage loans	12,600		13,129
Specific credit losses	22		45
Non-specifically identified credit losses	30		36
Valuation allowances	52	59	81	82	79	88
Mortgage loan, net of valuation allowance	12,548		13,048
Residential mortgage loans portfolio segment [Member]
Mortgage Loans on Real Estate [Line Items]
Evaluated individually for credit losses	13		13
Evaluated collectively for credit losses	805		676
Total mortgage loans	818		689
Specific credit losses	2		1
Non-specifically identified credit losses	0		1
Valuation allowances	2	2	2	19	18	14
Mortgage loan, net of valuation allowance	$ 816		$ 687