Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Proved Developed and Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2014 MMcfe		Dec. 31, 2013 MMcfe		Dec. 31, 2012 MMcfe
Reserve Quantities [Line Items]
Revisions	90,822		448,898		109,036
Extensions, discoveries and additions	2,398,709		1,732,944		1,767,202
Proved undeveloped reserves:
Proved undeveloped reserves	4,960,468		4,009,608
Natural Gas
Reserve Quantities [Line Items]
Beginning Balance	5,665,645		4,792,676		4,009,676
Revisions	(30,566)		384,825		76,925
Extensions, discoveries and additions	1,393,108		853,746		996,059
Purchases	262,813
Property sales	(81,238)		(101,074)		(73,429)
Production	(286,926)		(264,528)		(216,555)
Ending Balance	6,922,836		5,665,645		4,792,676
Proved developed reserves:
Proved developed reserves	3,583,051		2,797,483		2,373,604
Proved undeveloped reserves:
Proved undeveloped reserves	3,339,785		2,868,162		2,419,072
NGLs
Reserve Quantities [Line Items]
Beginning Balance	374,412		240,399		142,515
Revisions	19,716		7,743		3,036
Extensions, discoveries and additions	154,664		135,810		113,392
Purchases
Property sales	(14,064)		(286)		(11,575)
Production	(18,821)		(9,254)		(6,969)
Ending Balance	515,907		374,412		240,399
Proved developed reserves:
Proved developed reserves	270,271		206,477		154,984
Proved undeveloped reserves:
Proved undeveloped reserves	245,636		167,935		85,415
Crude Oil and Condensate
Reserve Quantities [Line Items]
Beginning Balance	48,360		45,082		31,532
Revisions	515		2,935		2,316
Extensions, discoveries and additions	12,936		10,723		15,131
Purchases
Property sales	(9,083)		(6,553)		(1,046)
Production	(4,070)		(3,827)		(2,851)
Ending Balance	48,658		48,360		45,082
Proved developed reserves:
Proved developed reserves	24,180		26,054		25,667
Proved undeveloped reserves:
Proved undeveloped reserves	24,478		22,306		19,415
Natural Gas Equivalents
Reserve Quantities [Line Items]
Beginning Balance	8,202,274	[1]	6,505,570	[1]	5,053,961	[1]
Revisions	90,822	[1]	448,898	[1]	109,036	[1]
Extensions, discoveries and additions	2,398,709	[1]	1,732,944	[1]	1,767,202	[1]
Purchases	262,813	[1]		[1]
Property sales	(220,122)	[1]	(142,116)	[1]	(149,153)	[1]
Production	(424,267)	[1]	(343,022)	[1]	(275,476)	[1]
Ending Balance	10,310,229	[1]	8,202,274	[1]	6,505,570	[1]
Proved developed reserves:
Proved developed reserves	5,349,761	[1]	4,192,666	[1]	3,457,502	[1]
Proved undeveloped reserves:
Proved undeveloped reserves	4,960,468	[1]	4,009,608	[1]	3,048,068	[1]

[1] Oil and NGLs are converted to mcfe at the rate of one barrel equals six mcf based upon the approximate relative energy content of oil to natural gas, which is not indicative of the relationship of oil and natural gas prices.