Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Proved Developed and Undeveloped Reserves (Detail)		12 Months Ended
		Dec. 31, 2015 MMcfe MMcf MBbls	Dec. 31, 2014 MMcfe MMcf MBbls	Dec. 31, 2013 MMcfe MMcf MBbls
Proved undeveloped reserves:
Proved undeveloped reserves | MMcfe		4,469,588	4,960,468
Natural Gas
Reserve Quantities [Line Items]
Beginning Balance | MMcf		6,922,836	5,665,645	4,792,676
Revisions | MMcf		(340,286)	(30,566)	384,825
Extensions, discoveries and additions | MMcf		1,017,956	1,393,108	853,746
Purchases | MMcf		0	262,813	0
Property sales | MMcf		(960,122)	(81,238)	(101,074)
Production | MMcf		(362,687)	(286,926)	(264,528)
Ending Balance | MMcf		6,277,697	6,922,836	5,665,645
Proved developed reserves:
Proved developed reserves | MMcf		3,376,165	3,583,051	2,797,483
Proved undeveloped reserves:
Proved undeveloped reserves | MMcf		2,901,533	3,339,785	2,868,162
NGLs
Reserve Quantities [Line Items]
Beginning Balance		515,907	374,412	240,399
Revisions		17,717	19,716	7,743
Extensions, discoveries and additions		36,308	154,664	135,810
Purchases		0	0	0
Property sales		(441)	(14,064)	(286)
Production		(20,356)	(18,821)	(9,254)
Ending Balance		549,135	515,907	374,412
Proved developed reserves:
Proved developed reserves		309,306	270,271	206,477
Proved undeveloped reserves:
Proved undeveloped reserves		239,828	245,636	167,935
Crude Oil and Condensate
Reserve Quantities [Line Items]
Beginning Balance		48,658	48,360	45,082
Revisions		3,804	515	2,935
Extensions, discoveries and additions		4,924	12,936	10,723
Purchases		0	0	0
Property sales		(109)	(9,083)	(6,553)
Production		(4,084)	(4,070)	(3,827)
Ending Balance		53,193	48,658	48,360
Proved developed reserves:
Proved developed reserves		31,679	24,180	26,054
Proved undeveloped reserves:
Proved undeveloped reserves		21,514	24,478	22,306
Natural Gas Equivalents
Proved developed reserves:
Proved developed reserves | MMcfe	[1]	5,422,075	5,349,761	4,192,666
Proved undeveloped reserves:
Proved undeveloped reserves | MMcfe	[1]	4,469,588	4,960,468	4,009,608
Beginning Balance | MMcfe	[1]	10,310,229	8,202,274	6,505,570
Revisions | MMcfe	[1]	(211,163)	90,822	448,898
Extensions, discoveries and additions | MMcfe	[1]	1,265,348	2,398,709	1,732,944
Purchases | MMcfe	[1]	0	262,813	0
Property sales | MMcfe	[1]	(963,423)	(220,122)	(142,116)
Production | MMcfe	[1]	(509,328)	(424,267)	(343,022)
Ending Balance | MMcfe	[1]	9,891,663	10,310,229	8,202,274

[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.