Summary of Significant Accounting Policies - Non-Cash Write-Downs of Carrying Value of Proved Oil and Gas Properties by Country (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	$ 25,517	$ 5,001	$ 995
Write-down of the carrying value of Oil and Gas properties, After tax	16,620	3,068	541
United States [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	19,537	4,412	552
Write-down of the carrying value of Oil and Gas properties, After tax	12,602	2,844	356
Canada [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	3,667
Write-down of the carrying value of Oil and Gas properties, After tax	2,721
North Sea [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	2,032	589	368
Write-down of the carrying value of Oil and Gas properties, After tax	1,016	$ 224	139
Egypt [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	281
Write-down of the carrying value of Oil and Gas properties, After tax	$ 281
Other International [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax			75
Write-down of the carrying value of Oil and Gas properties, After tax			$ 46