Net Quantities of Proved Reserves, Including Changes therein and Proved Developed and Proved Undeveloped Reserves (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2012 Well MBoe		Dec. 31, 2011 MBoe		Dec. 31, 2010 Location MBoe Well
Reserve Quantities [Line Items]
Revisions of previous estimates	(28,662,000)	[1]	(15,608,000)	[2]	(6,702,000)	[3]
Extensions, discoveries and other additions	18,255,000	[4]	19,792,000	[5]	19,142,000	[6]
Wells drilled on locations not classified as proved undeveloped					16
Added Proved undeveloped locations					51
Proved undeveloped Locations removed					200
Time period For development before revisions to reserves, max					5 years
Added drilling wells	18
Average oil and gas prices for time period	12 months
Discount rate	10.00%
2010 Revisions of Previous Estimates
Reserve Quantities [Line Items]
Revisions of previous estimates					(6,700)
Downward revisions due to removal of proved undeveloped locations that would not be developed within five years
Reserve Quantities [Line Items]
Revisions of previous estimates					(7,500)
Upward revisions as result of processing gas in Mid-Continent Granite Wash for NGLs
Reserve Quantities [Line Items]
Revisions of previous estimates					5,700
Upward revisions due to higher prices
Reserve Quantities [Line Items]
Revisions of previous estimates					2,000
Downward revisions as result of well performance, lease expirations and interest changes
Reserve Quantities [Line Items]
Revisions of previous estimates					(6,500)
2010 Extensions, Discoveries and Other Additions
Reserve Quantities [Line Items]
Extensions, discoveries and other additions					19,100
2011 Revisions of Previous Estimates
Reserve Quantities [Line Items]
Revisions of previous estimates			(15,600)
Downward revisions due to well performance issues, interest changes and economic limits
Reserve Quantities [Line Items]
Revisions of previous estimates			(12,000)
Downward revisions due to lower condensate yield in Granite Wash
Reserve Quantities [Line Items]
Revisions of previous estimates			(1,700)
Downward revisions due to elimination of proved undeveloped locations
Reserve Quantities [Line Items]
Revisions of previous estimates			(1,500)
Downward revisions due to lower natural gas prices
Reserve Quantities [Line Items]
Revisions of previous estimates			0
Upward revisions due to higher gas processing yields
Reserve Quantities [Line Items]
Revisions of previous estimates			500
2011 Extensions, Discoveries and Other Additions
Reserve Quantities [Line Items]
Extensions, discoveries and other additions			19,800
Increase due to drilling of additional wells and added proved undeveloped locations
Reserve Quantities [Line Items]
Extensions, discoveries and other additions			9,000
Increase due to drilling of additional wells and added proved undeveloped locations
Reserve Quantities [Line Items]
Extensions, discoveries and other additions			10,800
2012 Revisions of Previous Estimates
Reserve Quantities [Line Items]
Revisions of previous estimates	(28,700)
Downward revisions due to well performance issues, interest changes and economic limits
Reserve Quantities [Line Items]
Revisions of previous estimates	(5,000)
Downward revisions due to lower natural gas prices
Reserve Quantities [Line Items]
Revisions of previous estimates	(15,000)
Downward revisions due to the removal of proved undeveloped locations
Reserve Quantities [Line Items]
Revisions of previous estimates	(8,700)
2012 Extensions, Discoveries and Other Additions
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	18,300
Marcellus Shale and Selma Chalk
Reserve Quantities [Line Items]
Added Proved undeveloped locations			25
Other proved undeveloped locations
Reserve Quantities [Line Items]
Added Proved undeveloped locations			26
Selma Chalk, the Cotton Valley and the Haynesville Shale
Reserve Quantities [Line Items]
Proved undeveloped Locations removed	38
Marcellus Shale
Reserve Quantities [Line Items]
Added drilling wells			3
Granite Wash
Reserve Quantities [Line Items]
Added drilling wells			2
Eagle Ford Shale
Reserve Quantities [Line Items]
Added Proved undeveloped locations	48
Added drilling wells			28

[1] We had downward revisions of 28.7 MMBOE primarily as a result of the following: 1) downward revisions of 5.0 MMBOE due to well performance issues, interest changes and economic limits due to operating conditions, including lease operating expense and basis differentials, primarily in the Selma Chalk, the Granite Wash, the Cotton Valley, and the Haynesville and Marcellus Shales

[2] We had downward revisions of 15.6 MMBOE primarily as a result of the following: 1) downward revisions of 12.0 MMBOE due to well performance issues, interest changes and economic limits attributable to operating conditions particularly in the Granite Wash, Cotton Valley and Selma Chalk, 2) downward revisions of 1.7 MMBOE due to lower condensate yield in the Granite Wash, 3) downward revisions of 1.5 MMBOE attributable to the elimination of proved undeveloped locations particularly in the Haynesville Shale in East Texas, 4) downward revisions of 0.8 MMBOE due to lower natural gas prices and 5) upward revisions of 0.5 MMBOE due to higher gas processing yields in the Haynesville Shale and Granite Wash .

[3] We had downward revisions of 6.7 MMBOE primarily as a result of the following: 1) downward revisions of 7.5 MMBOE due to the removal of 200 proved undeveloped locations that would not be developed within five years, 2) upward revisions of 5.7 MMBOE as a result of processing the gas in the Mid-Continent Granite Wash for NGLs, 3) upward revisions of 2.0 MMBOE due to higher prices and 4) various downward revisions for 6.5 MMBOE across our assets as a result of well performance, lease expirations and interest changes.

[4] We added 18.3 MMBOE due primarily to the drilling of 18 wells and the addition of 48 proved undeveloped locations in the Eagle Ford Shale.

[5] We added 19.8 MMBOE due primarily to an increase of 9.0 MMBOE due to the drilling of three Marcellus Shale wells and two Granite Wash wells as well as the addition of 25 proved undeveloped locations in the Marcellus Shale and Selma Chalk. We also drilled 28 Eagle Ford Shale wells and added 26 proved undeveloped locations which resulted in an increase of 10.8 MMBOE .

[6] We added 19.1 MMBOE due to the drilling of 16 wells on locations not classified as proved undeveloped locations in our 2010 year-end reserve report and the addition of 51 new proved undeveloped locations, primarily in East Texas, as a result of our 2011 drilling activities .