Additional Financial Statement Information (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Production Expense
Lease Operating Expense			$ 571	$ 542	$ 563
Production and Ad Valorem Taxes			138	78	127
Gathering, Transportation and Processing Expense			432	480	306
Total			1,141	1,100	996
Exploration Expense
Leasehold Impairment and Amortization			62	148	113
Dry Hole Cost			9	579	266
Seismic, Geological and Geophysical			27	76	34
Staff Expense			55	77	43
Other			35	45	32
Exploration Expense			188	925	488
Loss on Marcellus Shale Upstream Divestiture
Loss on Sale			2,270	0	0
Firm Transportation Commitment			93	0	0
Other			16	0	0
Total			2,379	0	0
Other Operating (Income) Expense, Net
Marketing Expense			47	58	33
Clayton Williams Acquisition Expenses			100	0	0
Corporate Restructuring Expense			0	8	51
Pension Plan Expense			0	0	88
Impact of Rosetta Merger			0	(25)	81
North Sea Remediation Project Revision			(42)	0	0
Loss on Asset Due to Terminated Contract			0	41	0
Gain (loss) on disposition of assets			(326)	(238)	0
Other, Net			33	53	79
Total			(188)	(103)	332
Accounts Receivable, Net
Commodity Sales			455	403
Joint Interest Billings			207	106
Proceeds Receivable			0	40
Other			103	86
Allowance for Doubtful Accounts			(17)	(20)
Total			748	615
Other Current Assets
Inventories, Materials and Supplies			66	71
Inventories, Crude Oil			16	18
Assets Held for Sale			629	18
Restricted Cash			38	30
Prepaid Expenses and Other Assets, Current			31	23
Total			780	160
Other Noncurrent Assets
Equity Method Investments			305	400	$ 453
Mutual Fund Investments			57	71
Net Deferred Income Tax Asset			25	0
Other Assets, Noncurrent			74	37
Total			461	508
Other Current Liabilities
Production and Ad Valorem Taxes			84	115
Commodity Derivative Liabilities, Current			58	102
Income Taxes Payable			18	53
Asset Retirement Obligations, Current			51	160
Interest Payable			67	76
Compensation and Benefits Payable			98	110
Current Portion of Capital Lease and Other Obligations			61	63
Other Liabilities, Current			141	63
Total			578	742
Other Noncurrent Liabilities
Deferred Compensation Liabilities, Noncurrent			197	218
Asset Retirement Obligations, Noncurrent			824	775
Production and Ad Valorem Taxes			69	47
Marcellus Firm Transportation Commitment, Noncurrent			76	0
Other Liabilities, Noncurrent			79	63
Total			$ 1,245	$ 1,103
Ownership interest sold			35.00%	35.00%
Tamar and Dalit Fields
Other Noncurrent Liabilities
Ownership interest			7.50%
Adjustment | Gathering, Transportation and Processing Expense
Other Operating (Income) Expense, Net
Total	$ 17	$ 17