Summary of Significant Accounting Policies (Detail) - USD ($) $ in Millions			3 Months Ended			6 Months Ended
	Jan. 01, 2016	Dec. 31, 2015	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule Of Significant Accounting Policies [Line Items]
Asset impairment charges			$ 0		$ 173	$ 8	$ 173
U.K. Petroleum Revenue Tax rate	0.00%	35.00%
Impairment of certain GTP assets					105		105
Oil and gas property, fair value					143		143
Deferred tax assets			674			674
Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
Schedule Of Significant Accounting Policies [Line Items]
Deferred tax assets								$ 4
Additional Paid-in Capital [Member] | Accounting Standards Update 2016-09 [Member]
Schedule Of Significant Accounting Policies [Line Items]
Cumulative effect of new accounting principle								11
Accumulated Deficit [Member] | Accounting Standards Update 2016-09 [Member]
Schedule Of Significant Accounting Policies [Line Items]
Cumulative effect of new accounting principle								(11)
Accumulated Deficit [Member] | Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
Schedule Of Significant Accounting Policies [Line Items]
Cumulative effect of new accounting principle								$ 4
Gas Gathering, Transmission, and Processing Assets [Member]
Schedule Of Significant Accounting Policies [Line Items]
Oil and gas property, fair value					175		175
Oil and Gas Properties, Proved [Member]
Schedule Of Significant Accounting Policies [Line Items]
Impairments for oil and gas properties			0		68	0	68
Oil and Gas Properties, Unproved [Member]
Schedule Of Significant Accounting Policies [Line Items]
Impairments for oil and gas properties			$ 39		66	$ 54	108
Recoverable PRT Decommissioning Asset [Member]
Schedule Of Significant Accounting Policies [Line Items]
Asset impairment charges				$ 8
US and Canada [Member] | Oil and Gas Properties, Proved [Member]
Schedule Of Significant Accounting Policies [Line Items]
Impairments for oil and gas properties					$ 68		$ 68