Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
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	$ 4,300	$ 5,300		$ 9,600
Write-down of the carrying value of Oil and Gas properties, net of tax	2,800	3,400
Canada [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	835	1,400		2,200
Write-down of the carrying value of Oil and Gas properties, net of tax	617	1,000
North Sea [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties	663	632	$ 203	$ 1,300	$ 203
Write-down of the carrying value of Oil and Gas properties, net of tax	$ 331	$ 316	$ 77