Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Proved Developed and Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2012 Mcf MMcf	Dec. 31, 2011 Mcf MMcf	Dec. 31, 2010 Mcf
Reserve Quantities [Line Items]
Revisions	109,000,000	225,000,000	148,600,000
Extensions, discoveries and additions	1,800,000,000	1,500,000,000	1,400,000,000
Proved Undeveloped Reserves:
Proved undeveloped reserves	3,048,068	2,652,687
Natural Gas [Member]
Reserve Quantities [Line Items]
Beginning Balance	4,009,676	3,566,526	2,614,717
Revisions	76,925	73,643	3,599
Extensions, discoveries and additions	996,059	1,304,324	1,089,632
Purchases			124,981
Property sales	(73,429)	(777,816)	(124,369)
Production	(216,555)	(157,001)	(142,034)
Ending Balance	4,792,676	4,009,676	3,566,526
Proved Developed Reserves:
Proved developed reserves	2,373,604	1,907,209	1,762,766
Proved Undeveloped Reserves:
Proved undeveloped reserves	2,419,072	2,102,467	1,803,760
NGLs [Member]
Reserve Quantities [Line Items]
Beginning Balance	142,515	122,722	51,588
Revisions	3,036	18,627	26,832
Extensions, discoveries and additions	113,392	26,591	48,792
Purchases
Property sales	(11,575)	(19,852)
Production	(6,969)	(5,573)	(4,490)
Ending Balance	240,399	142,515	122,722
Proved Developed Reserves:
Proved developed reserves	154,984	64,472	53,071
Proved Undeveloped Reserves:
Proved undeveloped reserves	85,415	78,043	69,651
Crude Oil [Member]
Reserve Quantities [Line Items]
Beginning Balance	31,532	23,239	34,082
Revisions	2,316	6,522	(2,672)
Extensions, discoveries and additions	15,131	4,915	4,663
Purchases
Property sales	(1,046)	(1,176)	(10,865)
Production	(2,851)	(1,968)	(1,969)
Ending Balance	45,082	31,532	23,239
Proved Developed Reserves:
Proved developed reserves	25,667	17,872	17,050
Proved Undeveloped Reserves:
Proved undeveloped reserves	19,415	13,660	6,189
Natural Gas Equivalents [Member]
Reserve Quantities [Line Items]
Beginning Balance	5,053,961	4,442,290	3,128,739
Revisions	109,036	224,542	148,558
Extensions, discoveries and additions	1,767,202	1,493,357	1,410,359
Purchases			124,981
Property sales	(149,153)	(903,983)	(189,558)
Production	(275,476)	(202,245)	(180,789)
Ending Balance	6,505,570	5,053,961	4,442,290
Proved Developed Reserves:
Proved developed reserves	3,457,502	2,401,274	2,183,488
Proved Undeveloped Reserves:
Proved undeveloped reserves	3,048,068	2,652,687	2,258,802