Summary of Significant Accounting Policies - Recently Issued Accounting Standards (Detail) - USD ($) $ in Millions		Jan. 01, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets			$ 574	$ 603
Deferred income taxes			5,400	8,527
Short-term debt			33	0
Other Assets			2,350	2,310
Long-term Debt	[1]		$ 15,718	15,092
Accounting Standards Update 2015-17 [Member] | Adjustment Effect [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets				(722)
Deferred income taxes				$ (722)
Accounting Standards Update 2015-03 and 2015-15 [Member] | Adjustment Effect [Member] | Subsequent Event [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets		$ (1)
Short-term debt		(1)
Other Assets		(82)
Long-term Debt		$ (82)

[1] The total long-term debt balance for WES was $2.7 billion at December 31, 2015, and $2.4 billion at December 31, 2014.