Revenue Recognition - Disaggregated Revenue (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,233				$ 2,593
Losses on oil, natural gas and NGL derivatives	(251)				(368)
Revenues	2,255		$ 2,281		4,744	$ 5,034
Marcellus [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	169				463
Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	200				411
Eagle Ford [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	477				918
Utica [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	226				451
Mid-Continent [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	89				215
Powder River Basin [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	72				135
Oil [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	566				1,103
Losses on oil, natural gas and NGL derivatives	(202)				(288)
Oil [Member] | Marcellus [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0				0
Oil [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1				2
Oil [Member] | Eagle Ford [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	389				749
Oil [Member] | Utica [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	62				119
Oil [Member] | Mid-Continent [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	62				138
Oil [Member] | Powder River Basin [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	52				95
Natural gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	539				1,245
Losses on oil, natural gas and NGL derivatives	(35)				(67)
Natural gas [Member] | Marcellus [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	169				463
Natural gas [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	199				409
Natural gas [Member] | Eagle Ford [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	42				84
Natural gas [Member] | Utica [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	103				219
Natural gas [Member] | Mid-Continent [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	15				47
Natural gas [Member] | Powder River Basin [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	11				23
NGL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	128				245
Losses on oil, natural gas and NGL derivatives	(14)				(13)
NGL [Member] | Marcellus [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0				0
NGL [Member] | Haynesville [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0				0
NGL [Member] | Eagle Ford [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	46				85
NGL [Member] | Utica [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	61				113
NGL [Member] | Mid-Continent [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12				30
NGL [Member] | Powder River Basin [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	9				17
Oil, natrual gas and NGL [Member]
Disaggregation of Revenue [Line Items]
Revenues	982	$ 1,233	1,279	$ 1,079	2,225	2,748	$ 2,593	$ 2,226
Oil, natrual gas and NGL [Member] | Oil [Member]
Disaggregation of Revenue [Line Items]
Revenues	364				815
Oil, natrual gas and NGL [Member] | Natural gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	504				1,178
Oil, natrual gas and NGL [Member] | NGL [Member]
Disaggregation of Revenue [Line Items]
Revenues	114				232
Marketing [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,069				2,158
Revenues	1,273		$ 1,002		2,519	$ 2,286
Marketing [Member] | Oil [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	732				1,418
Revenues	877				1,680
Marketing [Member] | Natural gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	235				528
Revenues	294				627
Marketing [Member] | NGL [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	102				212
Revenues	102				212
Other marketing revenue [Member]
Disaggregation of Revenue [Line Items]
Revenues	204				361
Other marketing revenue [Member] | Oil [Member]
Disaggregation of Revenue [Line Items]
Revenues	145				262
Other marketing revenue [Member] | Natural gas [Member]
Disaggregation of Revenue [Line Items]
Revenues	59				99
Other marketing revenue [Member] | NGL [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 0				$ 0