Debt and Interest Expense - Debt Activity Table (Detail) $ in Millions	9 Months Ended
Sep. 30, 2015 USD ($)
Debt Disclosure [Abstract]
Balance at December 31, 2014	$ 15,071
Debt Instrument [Line Items]
Other, net	29
Balance at September 30, 2015	15,904
Commercial Paper [Member]
Debt Instrument [Line Items]
Borrowings, commercial paper	547
Commercial Paper [Member] \| Maturities Greater Than 90 Days [Member]
Debt Instrument [Line Items]
Repayments, commercial paper	$ (106)
7.50% Tangible Equity Units [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	1.50%
Debt instrument, maturity date	Jun. 07, 2018
$5.0 billion Facility [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowings, credit facility	$ 1,500
Repayments, credit facility	(1,500)
364-Day Credit Facility [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowings, credit facility	1,800
Repayments, credit facility	(1,800)
WES RCF [Member] \| Western Gas Partners, LP [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowings, credit facility	280
Repayments, credit facility	(610)
Senior Notes [Member] \| 7.50% Tangible Equity Units [Member]
Debt Instrument [Line Items]
Issuances	101
Repayments, senior notes	$ (8)
Senior Notes [Member] \| WES 3.950% Senior Notes Due 2025 [Member] \| Western Gas Partners, LP [Member]
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.95%
Issuances	$ 494
Debt instrument, maturity date	May 15, 2025

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Contractual interest rate for funds borrowed, under the debt agreement.

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