Goodwill and Other Intangible Assets - Goodwill (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line Items]
Goodwill	$ 5,395,000,000	$ 5,576,000,000	$ 5,492,000,000
Goodwill impairment loss	0
Oil and Gas Exploration and Production Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 4,945,000,000
Percentage of reporting unit fair value in excess of carrying amount	15.00%
WES Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 383,000,000
WES Transportation Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	5,000,000
Other Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 62,000,000