Supplemental Information on Natural Gas and Oil Exploration, Development and Production Activities (Unaudited) - Proved Developed and Undeveloped Reserves (Detail)		12 Months Ended
		Dec. 31, 2017 MMcfe MMcf MBbls	Dec. 31, 2016 MMcfe MMcf MBbls	Dec. 31, 2015 MMcfe MMcf MBbls
Proved undeveloped reserves:
Proved undeveloped reserves | MMcfe		6,914,287	5,302,414
Natural Gas
Reserve Quantities [Line Items]
Beginning Balance | MMcf		7,870,416	6,277,697	6,922,836
Revisions | MMcf		70,222	(7,441)	(340,286)
Extensions, discoveries and additions | MMcf		2,866,103	1,193,154	1,017,956
Purchases | MMcf		7,738	943,544
Property sales | MMcf		(60,278)	(160,727)	(960,122)
Production | MMcf		(490,552)	(375,811)	(362,687)
Ending Balance | MMcf		10,263,649	7,870,416	6,277,697
Proved developed reserves:
Proved developed reserves | MMcf		5,437,674	4,352,141	3,376,165
Proved undeveloped reserves:
Proved undeveloped reserves | MMcf		4,825,975	3,518,275	2,901,533
NGLs
Reserve Quantities [Line Items]
Beginning Balance		630,066	549,135	515,907
Revisions		83,338	41,402	17,717
Extensions, discoveries and additions		87,572	26,991	36,308
Purchases		330	40,724
Property sales		(2,356)	(360)	(441)
Production		(35,686)	(27,826)	(20,356)
Ending Balance		763,264	630,066	549,135
Proved developed reserves:
Proved developed reserves		448,258	363,852	309,306
Proved undeveloped reserves:
Proved undeveloped reserves		315,006	266,214	239,828
Crude Oil and Condensate
Reserve Quantities [Line Items]
Beginning Balance		70,252	53,193	48,658
Revisions		(10,555)	2,471	3,804
Extensions, discoveries and additions		15,997	6,506	4,924
Purchases		66	11,986
Property sales		(1,121)	(295)	(109)
Production		(4,785)	(3,609)	(4,084)
Ending Balance		69,854	70,252	53,193
Proved developed reserves:
Proved developed reserves		36,808	39,110	31,679
Proved undeveloped reserves:
Proved undeveloped reserves		33,046	31,143	21,514
Natural Gas Equivalents
Proved developed reserves:
Proved developed reserves | MMcfe	[1]	8,348,074	6,769,908	5,422,075
Proved undeveloped reserves:
Proved undeveloped reserves | MMcfe	[1]	6,914,287	5,302,414	4,469,588
Beginning Balance | MMcfe	[1]	12,072,322	9,891,663	10,310,229
Revisions | MMcfe	[1]	506,919	255,794	(211,163)
Extensions, discoveries and additions | MMcfe	[1]	3,487,519	1,394,134	1,265,348
Purchases | MMcfe	[1]	10,116	1,259,806
Property sales | MMcfe	[1]	(81,133)	(164,655)	(963,423)
Production | MMcfe	[1]	(733,382)	(564,420)	(509,328)
Ending Balance | MMcfe	[1]	15,262,361	12,072,322	9,891,663

[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.