Acquisitions, Divestitures, and Assets Held for Sale - Divestitures and Assets Held for Sale (Detail) - USD ($) $ in Millions		1 Months Ended	3 Months Ended				9 Months Ended
	Mar. 17, 2017	Oct. 31, 2017	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures							$ 3,480		$ 1,281
Gain (loss) on divestitures							(815)	[1]	516
Certain U.S. Onshore Assets in Moxa Arch, Wyoming [Member] | Held for Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term assets associated with assets held for sale				$ 557			557
Long-term liabilities associated with assets held for sale				37			37
Exploration and Production [Member] | Certain U.S. Onshore Assets in Eagleford, South Texas [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures							2,100
Gain (loss) on divestitures							(730)
Exploration and Production [Member] | Certain U.S. Onshore Assets in Eaglebine, Southeast Texas [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures							533
Gain (loss) on divestitures							(282)
Exploration and Production [Member] | Certain U.S. Onshore Assets in CBM, Utah [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures							69
Gain (loss) on divestitures							52
Exploration and Production [Member] | Certain U.S. Onshore Assets in Moxa Arch, Wyoming [Member] | Held for Sale [Member] | Fair Value, Inputs, Level 2 [Member] | Market Approach Valuation Technique [Member] | Fair Value, Measurements, Nonrecurring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on asset held for sale				(197)
Exploration and Production [Member] | Certain U.S. Onshore Assets in Wamsutter, Wyoming [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures									588
Gain (loss) on divestitures									59
Exploration and Production [Member] | Certain U.S. Onshore Assets in East Chalk, East Texas/Louisiana [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures									99
Gain (loss) on divestitures									(13)
Exploration and Production [Member] | Certain U.S. Onshore Assets in Elm Grove, East Texas/Louisiana [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures									100
Gain (loss) on divestitures									54
Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Marcellus, Pennsylvania [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures							758
Gain (loss) on divestitures							56
Proceeds receivable				$ 196			196
Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Marcellus, Pennsylvania [Member] | Asset Disposed of by Sale [Member] | Fair Value, Inputs, Level 2 [Member] | Market Approach Valuation Technique [Member] | Fair Value, Measurements, Nonrecurring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on asset held for sale					$ (129)
Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Ozona and Steward, West Texas [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from divestitures									223
Gain (loss) on divestitures									$ 50
Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Carthage, East Texas [Member] | Held for Sale [Member] | Fair Value, Inputs, Level 2 [Member] | Market Approach Valuation Technique [Member] | Fair Value, Measurements, Nonrecurring [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Loss on asset held for sale						$ (355)
Delaware Basin Joint Venture System [Member] | Western Gas Partners, LP [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on acquisition	$ 126						$ 126
Subsequent Event [Member] | Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Marcellus, Pennsylvania [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment		$ 193
Forecast [Member] | Exploration And Production Reporting Segment And Midstream Reporting Segment [Member] | Certain U.S. Onshore Assets in Marcellus, Pennsylvania [Member] | Asset Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Property, Plant, and Equipment			$ 3

[1] Includes the $126 million gain related to the property exchange discussed above.