Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Schedule Of Significant Accounting Policies [Line Items]
Discount rate	10.00%
United States [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	5,200
Write-down of the carrying value of Oil and Gas properties, net of tax	3,400
Canada [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	1,400
Write-down of the carrying value of Oil and Gas properties, net of tax	1,000
North Sea [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	632
Write-down of the carrying value of Oil and Gas properties, net of tax	316