Debt (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt disclosure
Total short-term debt	$ 600,000,000	$ 350,000,000
Total long-term debt	5,761,000,000	6,269,000,000
Total debt principal	6,361,000,000	6,619,000,000
Unamortized fair value adjustment, debt	52,000,000	53,000,000
Unamortized debt issuance costs	(63,000,000)	(67,000,000)
Total debt	6,350,000,000	6,605,000,000
Amount of commercial paper program	800,000,000
Junior subordinated debentures purchased by business trusts, description	The Company's three other junior subordinated debenture instruments are all similar in nature to each other. Three separate business trusts issued preferred securities to investors and used the proceeds to purchase the Company's subordinated debentures. Interest on each of the instruments is paid semi-annually.
Impact of amortization of the fair value adjustment on interest expense	1,000,000	1,000,000
Amount of debt obligations, other than commercial paper, due in 2013	500,000,000
Amount of debt obligations, other than commercial paper, due in 2015	400,000,000
Amount of debt obligations, other than commercial paper, due in 2016	400,000,000
Amount of debt obligations, other than commercial paper, due in 2017	450,000,000
Credit agreement, term (in years)	3 years
Credit agreement, maximum borrowing capacity	1,000,000,000
Credit agreement, threshold of consolidated net worth	$ 14,350,000,000
Credit agreement, covenant terms	The Company must also maintain a ratio of total debt to the sum of total debt plus consolidated net worth of not greater than 0.40 to 1.00. In addition, the credit agreement contains other customary restrictive covenants as well as certain customary events of default, including with respect to a change in control, which is defined to include the acquisition of 35% or more of the Company's voting stock and certain changes in the composition of the Company's board of directors. At December 31, 2012, the Company was in compliance with these covenants.
Credit agreement, cost of borrowing	Generally, the cost of borrowing under this agreement will range from LIBOR plus 100 basis points to LIBOR plus 175 basis points depending on the Company's credit ratings. At December 31, 2012, that cost would have been LIBOR plus 125 basis points had there been any amounts outstanding under the credit agreement.