Goodwill and Other Intangible Assets - Goodwill (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment loss	$ 0
Goodwill [Line Items]
Goodwill	5,576,000,000	5,492,000,000	5,492,000,000
Oil and Gas Exploration and Production Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	5,100,000,000
Other Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	69,000,000
WES Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	379,000,000
WES Transportation Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 5,000,000