Basis of Presentation - Statements of Operations Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Production Expense
Lease Operating Expense	$ 124	$ 151	$ 411	$ 414
Production and Ad Valorem Taxes	47	31	151	104
Gathering, Transportation and Processing Expense	97	93	292	333
Total	273	280	886	866
Exploration Expense
Leasehold Impairment and Amortization	0	33	0	51
Seismic, Geological and Geophysical	4	7	17	20
Staff Expense	14	11	41	40
Other	7	13	31	25
Total	25	64	89	136
Other Operating Expense (Income), Net
Marketing Expense	11	6	21	39
Purchased Oil and Gas	76	0	204	0
Clayton Williams Energy Acquisition Expenses	0	4	0	98
Gain on Asset Retirement Obligation Revisions	(10)	(42)	(21)	(42)
Other, Net	1	17	18	37
Total	78	(15)	222	132
Other Non-Operating (Income) Expense, Net
Gain on Investment in Shares of Tamar Petroleum Ltd., Net	(32)	0	(6)	0
Other, Net	(2)	2	(1)	(4)
Loss (Gain) on Extinguishment of Debt, Net	0	98	(3)	98
Total	(34)	100	(10)	94
Dividend income	17		31
Other Royalty Expense
Production Expense
Other Royalty Expense	5	5	32	15
Total
Sales of Purchased Oil and Gas and Other
Revenue from Sales	137	53	380	137
Sales of Purchased Oil and Gas
Sales of Purchased Oil and Gas and Other
Revenue from Sales	72	0	191	0
Income from Equity Method Investees
Sales of Purchased Oil and Gas and Other
Revenue from Sales	44	46	140	125
Midstream Services Revenues – Third Party
Sales of Purchased Oil and Gas and Other
Revenue from Sales	$ 21	$ 7	$ 49	$ 12