Goodwill and Other Intangible Assets - Goodwill (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill impairment loss	$ 0
Goodwill [Line Items]
Goodwill	5,492,000,000	5,641,000,000	5,282,000,000
Oil and Gas Exploration and Production Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	5,317,000,000
Other Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	87,000,000
WES Gathering and Processing Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	83,000,000
Transportation Reporting Unit [Member]
Goodwill [Line Items]
Goodwill	$ 5,000,000