Goodwill and Other Intangible Assets - Goodwill (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 4,789,000,000	$ 5,000,000,000	$ 5,395,000,000
Decrease in goodwill during the year	211,000,000
Goodwill impairment loss	0
Oil and Gas Exploration and Production Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	4,300,000,000
WES Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	411,000,000
WES Transportation Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	5,000,000
Other Midstream Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 30,000,000