Basis of Presentation - Statements of Operations Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income From Equity Method Investees and Other
Income from Equity Method Investees	$ 49	$ 38	$ 96	$ 80
Sales of Purchased Oil and Gas	66	0	119	0
Midstream Services Revenues – Third Party	15	4	28	4
Total	130	42	243	84
Production Expense
Lease Operating Expense	132	124	287	263
Production and Ad Valorem Taxes	50	32	104	73
Gathering, Transportation and Processing Expense	100	121	195	240
Total	292	283	613	586
Exploration Expense
Leasehold Impairment and Amortization	0	0	0	18
Seismic, Geological and Geophysical	2	8	13	13
Staff Expense	13	16	27	29
Other	14	6	24	12
Total	29	30	64	72
Other Operating Expense, Net
Marketing Expense	7	14	12	33
Purchased Oil and Gas	71	0	128	0
Clayton Williams Energy Acquisition Expenses	0	90	0	94
Other, Net	(4)	14	4	20
Total	74	118	144	147
Other Non-Operating Expense (Income), Net
Loss on Investment in Tamar Petroleum Ltd., Net	11	0	26	0
Other	0	(5)	(2)	(6)
Total	11	(5)	24	(6)
Loss from change in fair value	11		40
Dividend income			14
Saddle Butte
Other Operating Expense, Net
Purchased Oil and Gas	6		11
Other Royalty Expense
Production Expense
Other Royalty Expense	$ 10	$ 6	$ 27	$ 10