Subsequent Events - Additional Information (Detail) (USD $)	1 Months Ended
	Oct. 16, 2012
Separate U.S. downstream business into an independent company
Subsequent Event [Line Items]
Estimated completion date	2013
Dividend declared
Subsequent Event [Line Items]
Special dividend payable amount, per share	2.50
Special dividend declared date	Oct. 16, 2012
Special dividend payment date	Dec. 03, 2012
Special dividend record date	Nov. 16, 2012
Special dividend paid	486,000,000
Stock buyback program | Maximum
Subsequent Event [Line Items]
Stock buyback program	1,000,000,000