Debt and Interest Expense - Outstanding Debt Table (Detail) (USD $) In Millions, unless otherwise specified						12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2012 Senior Notes, 6 Point 125 Percent, Due 2012 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 125 Percent, Due 2012 [Member]	Dec. 31, 2012 Senior Notes, 5 Point 000 Percent, Due 2012 [Member]	Dec. 31, 2011 Senior Notes, 5 Point 000 Percent, Due 2012 [Member]	Dec. 31, 2012 Senior Notes, 5 Point 750 Percent, Due 2014 [Member]	Dec. 31, 2011 Senior Notes, 5 Point 750 Percent, Due 2014 [Member]	Dec. 31, 2012 Senior Notes, 7 Point 625 Percent, Due 2014 [Member]	Dec. 31, 2011 Senior Notes, 7 Point 625 Percent, Due 2014 [Member]	Dec. 31, 2012 Senior Notes, 5 Point 950 Percent, Due 2016 [Member]	Dec. 31, 2011 Senior Notes, 5 Point 950 Percent, Due 2016 [Member]	Dec. 31, 2012 Senior Notes, 6 Point 375 Percent, Due 2017 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 375 Percent, Due 2017 [Member]	Dec. 31, 2012 Debentures, 7 Point 050 Percent, Due 2018 [Member]	Dec. 31, 2011 Debentures, 7 Point 050 Percent, Due 2018 [Member]	Dec. 31, 2012 Senior Notes, 6 Point 950 Percent, Due 2019 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 950 Percent, Due 2019 [Member]	Dec. 31, 2012 Senior Notes, 8 Point 700 Percent, Due 2019 [Member]	Dec. 31, 2011 Senior Notes, 8 Point 700 Percent, Due 2019 [Member]	Dec. 31, 2012 Senior Notes, 5 Point 375 Percent, Due 2021 [Member] Western Gas Partners Limited Partnership [Member]	Dec. 31, 2011 Senior Notes, 5 Point 375 Percent, Due 2021 [Member] Western Gas Partners Limited Partnership [Member]	Dec. 31, 2012 Senior Notes, 4 Point 000 Percent, Due 2022 [Member] Western Gas Partners Limited Partnership [Member]	Dec. 31, 2012 Senior Notes, 6 Point 950 Percent, Due 2024 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 950 Percent, Due 2024 [Member]	Dec. 31, 2012 Debentures, 7 Point 500 Percent, Due 2026 [Member]	Dec. 31, 2011 Debentures, 7 Point 500 Percent, Due 2026 [Member]	Dec. 31, 2012 Debentures, 7 Point 000 Percent Due 2027 [Member]	Dec. 31, 2011 Debentures, 7 Point 000 Percent Due 2027 [Member]	Dec. 31, 2012 Debentures, 7 Point 125 Percent, Due 2027 [Member]	Dec. 31, 2011 Debentures, 7 Point 125 Percent, Due 2027 [Member]	Dec. 31, 2012 Debentures, 6 Point 625 Percent, Due 2028 [Member]	Dec. 31, 2011 Debentures, 6 Point 625 Percent, Due 2028 [Member]	Dec. 31, 2012 Debentures, 7 Point 150 Percent, Due 2028 [Member]	Dec. 31, 2011 Debentures, 7 Point 150 Percent, Due 2028 [Member]	Dec. 31, 2012 Debentures, 7 Point 200 Percent, Due 2029 [Member]	Dec. 31, 2011 Debentures, 7 Point 200 Percent, Due 2029 [Member]	Dec. 31, 2012 Debentures, 7 Point 950 Percent, Due 2029 [Member]	Dec. 31, 2011 Debentures, 7 Point 950 Percent, Due 2029 [Member]	Dec. 31, 2012 Senior Notes, 7 Point 500 Percent, Due 2031 [Member]	Dec. 31, 2011 Senior Notes, 7 Point 500 Percent, Due 2031 [Member]	Dec. 31, 2012 Senior Notes, 7 Point 875 Percent, Due 2031 [Member]	Dec. 31, 2011 Senior Notes, 7 Point 875 Percent, Due 2031 [Member]	Dec. 31, 2012 Senior Notes, Zero Coupon due 2036 [Member]	Dec. 31, 2011 Senior Notes, Zero Coupon due 2036 [Member]	Dec. 31, 2012 Senior Notes, 6 Point 450 Percent, Due 2036 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 450 Percent, Due 2036 [Member]	Dec. 31, 2012 Senior Notes, 7 Point 950 Percent, Due 2039 [Member]	Dec. 31, 2011 Senior Notes, 7 Point 950 Percent, Due 2039 [Member]	Dec. 31, 2012 Senior Notes, 6 Point 200 Percent, Due 2040 [Member]	Dec. 31, 2011 Senior Notes, 6 Point 200 Percent, Due 2040 [Member]	Dec. 31, 2012 Debentures, 7 Point 730 Percent, Due 2096 [Member]	Dec. 31, 2011 Debentures, 7 Point 730 Percent, Due 2096 [Member]	Dec. 31, 2012 Debentures, 7 Point 500 Percent, Due 2096 [Member]	Dec. 31, 2011 Debentures, 7 Point 500 Percent, Due 2096 [Member]	Dec. 31, 2012 Debentures, 7 Point 250 Percent, Due 2096 [Member]	Dec. 31, 2011 Debentures, 7 Point 250 Percent, Due 2096 [Member]	Dec. 31, 2011 Five Billion Dollar Facility [Member]
Debt Instrument [Line Items]
Principal	$ 14,952		$ 16,952				$ 131		$ 39	$ 275	$ 275	$ 500	$ 500	$ 1,750	$ 1,750	$ 2,000	$ 2,000	$ 114	$ 114	$ 300	$ 300	$ 600	$ 600	$ 500	$ 500	$ 670	$ 650	$ 650	$ 112	$ 112	$ 54	$ 54	$ 150	$ 150	$ 17	$ 17	$ 235	$ 235	$ 135	$ 135	$ 117	$ 117	$ 900	$ 900	$ 500	$ 500	$ 2,360	$ 2,360	$ 1,750	$ 1,750	$ 325	$ 325	$ 750	$ 750	$ 61	$ 61	$ 78	$ 78	$ 49	$ 49	$ 2,500
Debt instrument, stated interest rate						6.125%		5.00%		5.75%		7.625%		5.95%		6.375%		7.05%		6.95%		8.70%		5.375%		4.00%	6.95%		7.50%		7.00%		7.125%		6.625%		7.15%		7.20%		7.95%		7.50%		7.875%				6.45%		7.95%		6.20%		7.73%		7.50%		7.25%
Debt instrument, maturity date						Mar. 15, 2012		Oct. 01, 2012		Jun. 15, 2014		Mar. 15, 2014		Sep. 15, 2016		Sep. 15, 2017		May 15, 2018		Jun. 15, 2019		Mar. 15, 2019		Jun. 01, 2021		Jul. 01, 2022	Jul. 01, 2024		Oct. 15, 2026		Nov. 15, 2027		Oct. 15, 2027		Jan. 15, 2028		May 15, 2028		Mar. 15, 2029		Apr. 15, 2029		May 01, 2031		Sep. 15, 2031		Oct. 10, 2036		Sep. 15, 2036		Jun. 15, 2039		Mar. 15, 2040		Sep. 15, 2096		Nov. 01, 2096		Nov. 15, 2096
Net unamortized discounts and premiums	(1,683)	[1]	(1,722)	[1]
Total borrowings	13,269		15,230		12,787
Less current portion of long-term debt			170
Total long-term debt	$ 13,269		$ 15,060

[1] Unamortized discounts and premiums are amortized over the term of the related debt.