Debt - Additional Information (Detail) (USD $)	12 Months Ended				1 Months Ended						12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 09, 2011	Sep. 09, 2011 2.13% Notes Due 2016	Sep. 09, 2011 3.35% Notes Due 2021	Sep. 09, 2011 4.85% Notes Due 2041	Oct. 31, 2011 Due In 2013	Aug. 31, 2011 New Revolving Credit Facility	Aug. 31, 2011	Dec. 31, 2012 4.07% maturing in December 2042	Dec. 31, 2012 5.50% to 8.50% maturing in 2023 to 2040	Dec. 31, 2012 5.50% to 8.50% maturing in 2023 to 2040 Minimum	Dec. 31, 2012 5.50% to 8.50% maturing in 2023 to 2040 Maximum
Debt Instrument [Line Items]
Amount of issued debt				$ 2,000,000,000	$ 500,000,000	$ 900,000,000	$ 600,000,000				$ 1,300,000,000
Senior note, interest rate					2.13%	3.35%	4.85%				4.07%		5.50%	8.50%
Senior note, maturity date											2042-12
Principal amount of debt exchanged												1,200,000,000
Senior note, maturity date					2016	2021	2041	2013					2023	2040
Premium paid for debt exchange	393,000,000
New notes issued for debt exchange											168,000,000
Cash premium for debt exchange	225,000,000
Unamortized discounts	892,000,000	506,000,000										194,000,000
Redemption of debt		84,000,000						500,000,000
Premiums paid in connection with early extinguishments of debt		48,000,000
Revolving credit facility									1,500,000,000	1,500,000,000
Revolving credit facility expiration date									2016-08	2012-06
Additional amount to increase credit facility, at banks' discretion	500,000,000
Long-term debt, due in next 12 months	150,000,000
Long-term debt, due in year four	952,000,000
Interest payments	$ 378,000,000	$ 326,000,000	$ 337,000,000