Variable Interest Entities (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Variable Interest Entity [Line Items]
Interest income for WES	$ 251	$ 245	$ 768	$ 743
Variable Interest Entity, Primary Beneficiary [Member] | Western Gas Partners, LP [Member]
Variable Interest Entity [Line Items]
Note receivable for WES	$ 260		$ 260
Debt instrument, term			30 years
Fixed annual rate	6.50%		6.50%
Interest income for WES	$ 5	$ 5	$ 13	13
Deferred purchase price obligation - Anadarko	$ 16		16		$ 189
Above-market component of swap extensions with Anadarko			$ 35	$ 8