Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Summary Of Significant Accounting Policies [Line Items]
Discount rate	10.00%
Write-down of the carrying value of Oil and Gas properties	$ 65	$ 521
Argentina [Member]
Summary Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	65
Write-down of the carrying value of Oil and Gas properties, net of tax	42
Canada [Member]
Summary Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties		521
Write-down of the carrying value of Oil and Gas properties, net of tax		$ 390