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Outstanding Debt and Capital Lease Obligations Table (Detail) (USD $) In Millions	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Total borrowings	$ 12,720	$ 12,998	$ 12,966	$ 12,787
Capital lease obligations	229			226
Less: Current portion of long-term debt	141			291
Total long-term debt	12,808			12,722
Long-term notes and debentures [Member]
Debt Instrument [Line Items]
Principal	13,952			14,237
Total borrowings	12,226			12,488
Fair Value	13,752			13,459
Western Gas Partners Limited Partnership [Member] \| Total borrowings
Debt Instrument [Line Items]
Principal	500			299
Fair Value	503			299
Total borrowings
Debt Instrument [Line Items]
Principal	14,452	14,737	14,707	14,536
Fair Value	14,255			13,758
Capital lease obligations [Member]
Debt Instrument [Line Items]
Principal	229			226
Current portion of long-term debt [Member]
Debt Instrument [Line Items]
Principal	141			289
Fair Value	134			296
Noncurrent portion of long-term debt [Member]
Debt Instrument [Line Items]
Principal	14,540			14,473
Fair Value	14,121			13,462
Western Gas Partners Limited Partnership [Member]
Debt Instrument [Line Items]
Total borrowings	$ 494			$ 299