Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from external customers		$ 816	$ 811	$ 1,736	$ 1,657
Depreciation, depletion and amortization expense		142	123	285	229
Operating income		124	188	379	454
Interest expense		32	34	71	66
Gain (loss) on derivatives		(36)	134	(43)	250
Loss on early extinguishment of debt	$ (59)	(8)	(10)	(8)	(11)
Other income (loss), net		3	6	2	8
Assets		7,042	7,150	7,042	7,150	$ 7,521
Capital investments		403	325	741	615
Restructuring charges		18		18
Cash and cash equivalents		37	1,111	37	1,111	$ 916	$ 1,423
Change in accrued expenditures		19	41	52	(11)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(501)	(537)	(1,114)	(1,112)
E&P [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		527	531	1,170	1,097
Depreciation, depletion and amortization expense		126	107	243	197
Operating income		97	146	335	371
Interest expense		32	34	71	66
Gain (loss) on derivatives		(36)	134	(43)	250
Other income (loss), net		3	2	3	4
Assets		5,583	4,633	5,583	4,633
Capital investments		396	318	730	601
Restructuring charges		16		16
E&P [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		7	5	13	8
Midstream [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		289	280	566	560
Depreciation, depletion and amortization expense		16	16	42	32
Operating income		27	42	44	83
Other income (loss), net			4	(1)	4
Assets		1,228	1,281	1,228	1,281
Capital investments		5	6	9	12
Impairment				10
Restructuring charges		2		2
Midstream [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(508)	(542)	(1,127)	(1,120)
Other [Member]
Segment Reporting Information [Line Items]
Loss on early extinguishment of debt		(8)	(10)	(8)	(11)
Assets		231	1,236	231	1,236
Capital investments		2	1	2	2
Cash and cash equivalents		37	1,100	37	1,100
Workforce Reduction [Member]
Segment Reporting Information [Line Items]
Restructuring charges		18		18
Workforce Reduction [Member] | E&P [Member]
Segment Reporting Information [Line Items]
Restructuring charges		16		16
Workforce Reduction [Member] | Midstream [Member]
Segment Reporting Information [Line Items]
Restructuring charges		2		2
Marketing [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		265	250	518	503
Marketing [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(463)	(490)	(1,039)	(1,014)
Marketing [Member] | Midstream [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		$ 463	$ 490	$ 1,039	$ 1,014