Segment Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues		$ 809	$ 737	$ 811	$ 846	$ 684	$ 651	$ 522	$ 579		$ 3,203	$ 2,436	$ 3,133
Depreciation, depletion and amortization expense											504	436	1,091
Impairment of natural gas and oil properties						0	817	470	1,034		0	2,321	6,950
Operating income (loss)		167	$ 110	$ 188	$ 266	122	$ (725)	$ (492)	$ (1,100)		731	(2,195)	(6,522)
Interest expense											135	88	56
Gain (loss) on derivatives											422	(339)	47
Loss on extinguishment of debt	$ 59									$ (11)	(70)	(51)
Other income (loss), net											5	1	(30)
Provision (benefit) for income taxes											(93)	(29)	(2,005)
Assets		7,521				7,076					7,521	7,076	8,086
Capital investments											1,293	648	2,437
Restructuring charges												78
Cash and cash equivalents		916				1,423					916	1,423	15	$ 53
Change in accrued expenditures												43	(33)
Gain on sale of assets, net											6		283
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											(2,081)	(1,546)	(2,060)
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenues											2,105	1,435	2,095
Depreciation, depletion and amortization expense											440	371	1,028
Impairment of natural gas and oil properties												2,321	6,950
Operating income (loss)											549	(2,404)	(7,104)
Interest expense											135	87	47
Gain (loss) on derivatives											421	(338)	51
Other income (loss), net											4	5	(21)
Provision (benefit) for income taxes											(93)	(29)	(2,273)
Assets		5,109				4,178					5,109	4,178	6,588
Capital investments											1,248	623	2,258
Restructuring and other one-time charges												86
Exploration and Production [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											19	22	21
Midstream Services [Member]
Segment Reporting Information [Line Items]
Revenues											1,098	1,001	1,038
Depreciation, depletion and amortization expense											64	65	62
Operating income (loss)											183	209	583
Interest expense												1	9
Gain (loss) on derivatives											1	(1)
Other income (loss), net											1	(2)	(9)
Provision (benefit) for income taxes													268
Assets		1,288				1,331					1,288	1,331	1,290
Capital investments											32	21	167
Gain on sale of assets, net													277
Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											(2,100)	(1,568)	(2,081)
Other [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion and amortization expense													1
Operating income (loss)											(1)		(1)
Gain (loss) on derivatives													(4)
Loss on extinguishment of debt											(70)	(51)
Other income (loss), net												(2)
Assets		$ 1,124				$ 1,567					1,124	1,567	208
Capital investments											13	4	12
Marketing [Member] | Midstream Services [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues											$ (1,900)	(1,300)	$ (1,800)
Workforce Reduction [Member] | Exploration and Production [Member]
Segment Reporting Information [Line Items]
Restructuring charges												75
Workforce Reduction [Member] | Midstream Services [Member]
Segment Reporting Information [Line Items]
Restructuring charges												$ 3