Basis of Presentation (Details) $ in Millions		3 Months Ended			6 Months Ended	9 Months Ended
	Jun. 30, 2017 MMBoe	Sep. 30, 2017 USD ($) MMBoe	Jun. 30, 2017 USD ($) MMBoe	Sep. 30, 2016 USD ($)	Jun. 30, 2017 MMBoe	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Apr. 24, 2017 USD ($)	Dec. 31, 2016 USD ($)
Goodwill		$ 1,295				$ 1,295			$ 0
Proved undeveloped reserves (MMBOEs) | MMBoe	551		551		551
Production Expense
Lease Operating Expense		151		$ 131		414	$ 412
Production and Ad Valorem Taxes		36		30		119	73
Gathering, Transportation and Processing Expense		93		121		333	354
Total		280		282		866	839
Exploration Expense
Leasehold Impairment and Amortization		33		96		51	127
Dry Hole Cost		2		5		2	105
Seismic, Geological and Geophysical		7		15		20	47
Staff Expense		11		15		40	53
Other		11		(6)		23	44
Total		64		125		136	376
Loss on Sale		0		0		2,270	0
Firm transportation liability		0		0		41	0
Other		4		0		15	0
Total		4		0		2,326	0
Other Operating Expense, Net (7)
Marketing Expense		6		12		39	39
Clayton Williams Energy Acquisition Expenses		4		0		98	0
Loss on Asset Due to Terminated Contract		0		0		0	47
North Sea Remediation Project Revision		(42)		0		(42)	0
Other, Net		17		25		37	41
Total		(15)		37		132	127
Gathering, Transportation and Processing Expense		93		121		333	354
Accounts Receivable, Net
Commodity Sales		403				403			403
Joint Interest Billings		183				183			106
Proceeds Receivable		0				0			40
Other		106				106			86
Allowance for Doubtful Accounts		(17)				(17)			(20)
Total		675				675			615
Other Current Assets
Inventories, Materials and Supplies		61				61			71
Inventories, Crude Oil		17				17			18
Assets Held for Sale		180				180			18
Restricted Cash		0				0			30
Prepaid Expenses and Other Current Assets		45				45			23
Total		303				303			160
Other Noncurrent Assets
Equity Method Investments		286				286			400
Mutual Fund Investments		70				70			71
Other Assets, Noncurrent		60				60			37
Total		416				416			508
Other Current Liabilities
Production and Ad Valorem Taxes		118				118			115
Commodity Derivative Liabilities		4				4			102
Income Taxes Payable		13				13			53
Asset Retirement Obligations		50				50			160
Interest Payable		82				82			76
Current Portion of Capital Lease Obligations		65				65			63
Foreign Sales Tax Payable		29				29			14
Compensation and Benefits Payable		87				87			110
Theoretical Withdrawal Premium		25				25			18
Other Liabilities, Current		26				26			31
Total		499				499			742
Other Noncurrent Liabilities
Deferred Compensation Liabilities		216				216			218
Asset Retirement Obligations		894				894			775
Marcellus Shale Firm Transportation Commitment		31				31			0
Production and Ad Valorem Taxes		49				49			47
Other Liabilities, Noncurrent		55				55			63
Total		1,245				1,245			$ 1,103
Percentage of divestiture farmed out									35.00%
Firm transportation liability		$ 39				39
Leviathan
Proved undeveloped reserves (MMBOEs) | MMBoe	551		551		551
Tamar Field
Increase (decrease) in net proved reserves (MMBOEs) | MMBoe		48
Marcellus Shale
Proved developed and undeveloped reserves, decreased reserves from divestiture (MMBOEs) | MMBoe	241
Decrease in proved developed reserves (MMBOEs) | MMBoe	190
Decrease in undeveloped reserves (MMBOEs) | MMBoe	51
Clayton Williams Energy
Goodwill		$ 1,300				1,300		$ 1,295
Other Operating Expense, Net (7)
Clayton Williams Energy Acquisition Expenses			$ 23			64
Clayton Williams Energy | Delaware Basin
Increase (decrease) in net proved reserves (MMBOEs) | MMBoe					86
Proved developed reserves (MMBOEs) | MMBoe	17		17		17
Proved undeveloped reserves (MMBOEs) | MMBoe	69		69		69
Scenario, Previously Reported
Other Operating Expense, Net (7)
Gathering, Transportation and Processing Expense		12		$ 8		17	$ 19
Other Current Liabilities
Other Noncurrent Liabilities
Firm transportation liability		$ 8				$ 8
Texas Reporting Unit
Percentage of fair value in excess of carrying amount		6.00%				6.00%