Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended	9 Months Ended		3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 United States [Member]	Sep. 30, 2013 Argentina [Member]	Mar. 31, 2013 Argentina [Member]	Sep. 30, 2012 Canada [Member]	Jun. 30, 2012 Canada [Member]	Mar. 31, 2012 Canada [Member]	Sep. 30, 2013 Kenya [Member]
Summary of Significant Accounting Policies [Line Items]
Discount rate			10.00%
Write-down of the carrying value of Oil and Gas properties						$ 552	$ 116	$ 65	$ 721	$ 641	$ 521
Write-down of the carrying value of Oil and Gas properties, net of tax						356	76	42	539	480	390
Impairment of the carrying value	743	729		808	1,898							75
Impairment of the carrying value Net of tax												$ 46