Schedule of Production Payment (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012 Mcfe	Mar. 31, 2012 Andarko Basin Granite Wash [Member] Y Mcfe	May 31, 2012 Mid Continent Joint Venture [Member] Mcfe Y	Sep. 30, 2010 Barnett Shale Joint Venture [Member] Y Mcfe	Feb. 28, 2010 East Texas And Texas Gulf Coast [Member] Mcfe Y	Aug. 31, 2009 South Texas [Member] Y Mcfe	Dec. 31, 2008 Anadarko And Arkoma Basins [Member] Mcfe Y	Aug. 31, 2008 Anadarko Basin Woodford Shale [Member] Mcfe Y	Sep. 30, 2012 Texas, Oklahoma And Kansas Mcfe	May 31, 2008 Texas, Oklahoma And Kansas Mcfe Y	Dec. 31, 2007 Kentucky And West Virginia [Member] Mcfe Y	Jun. 01, 2010 Permian Basin
Business Combination, Separately Recognized Transactions [Line Items]
Proceeds	$ 6,031	$ 744	$ 853	$ 1,150	$ 180	$ 370	$ 412	$ 600		$ 622	$ 1,100
Proved Reserves (at time of sale) (bcfe)	1,334,000,000	160,000,000	177,000,000	390,000,000	46,000,000	68,000,000	98,000,000	93,000,000	28,000,000	94,000,000	208,000,000
$ / mcfe	4.52	4.68	4.82	2.93	3.95	5.46	4.19	6.38		6.53	5.29
Original Term (years)		10	10	5	10	7.5	8	11		11	15
Purchase												$ 313