Summary of Significant Accounting Policies - Non-Cash Write-Downs of Carrying Value Of Proved Oil And Gas Properties By Country (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	$ 488	$ 7,220
Write-down of the carrying value of Oil and Gas properties, After tax	325	4,704
U.S. [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	0	5,235
Write-down of the carrying value of Oil and Gas properties, After tax	0	3,377
Canada [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	0	1,353
Write-down of the carrying value of Oil and Gas properties, After tax	0	1,011
North Sea [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	325	632
Write-down of the carrying value of Oil and Gas properties, After tax	162	316
Egypt [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	163	0
Write-down of the carrying value of Oil and Gas properties, After tax	$ 163	$ 0