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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	$ 5,721	$ 1,562	$ 18,757	$ 1,765
Write-down of the carrying value of Oil and Gas properties, After tax	3,716	1,002	12,155	1,079
United States [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	4,301	1,545	13,855	1,545
Write-down of the carrying value of Oil and Gas properties, After tax	2,774	995	8,937	995
Canada [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	973		3,160
Write-down of the carrying value of Oil and Gas properties, After tax	719		2,347
North Sea [Member]
Schedule Of Significant Accounting Policies [Line Items]
Write-down of the carrying value of Oil and Gas properties, Before tax	447	17	1,742	220
Write-down of the carrying value of Oil and Gas properties, After tax	$ 223	$ 7	$ 871	$ 84