Derivative Activities - Schedule of Additional Information Related to Master Netting Arrangements with Derivative Counterparties (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Gross Amounts of Recognized Assets	$ 124,647	$ 77,079
Gross Amounts Offset in the Balance Sheet	(65,767)	(63,596)
Net Amounts of Assets Presented in the Balance Sheet	58,880	13,483
Gross Amounts of Recognized (Liabilities)	(119,789)	(253,096)
Gross Amounts Offset in the Balance Sheet	65,767	63,596
Net Amounts of (Liabilities) Presented in the Balance Sheet	(54,022)	(189,500)
Commodity | Swaps | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized Assets	87,794	13,213
Gross Amounts Offset in the Balance Sheet	(4,106)	(11,425)
Net Amounts of Assets Presented in the Balance Sheet	83,688	1,788
Gross Amounts of Recognized (Liabilities)	(216)	(158,359)
Gross Amounts Offset in the Balance Sheet	4,106	11,425
Net Amounts of (Liabilities) Presented in the Balance Sheet	3,890	(146,934)
Commodity | Swaps | Crude Oil
Derivative [Line Items]
Gross Amounts of Recognized Assets	2	9,356
Gross Amounts Offset in the Balance Sheet	(7,928)	(3,489)
Net Amounts of Assets Presented in the Balance Sheet	(7,926)	5,867
Gross Amounts of Recognized (Liabilities)	(24,726)	(13,206)
Gross Amounts Offset in the Balance Sheet	7,928	3,489
Net Amounts of (Liabilities) Presented in the Balance Sheet	(16,798)	(9,717)
Commodity | Swaps | NGLs (C2-Ethane)
Derivative [Line Items]
Gross Amounts of Recognized Assets	57	53
Gross Amounts Offset in the Balance Sheet	0	(53)
Net Amounts of Assets Presented in the Balance Sheet	57	0
Gross Amounts of Recognized (Liabilities)		(1,008)
Gross Amounts Offset in the Balance Sheet		53
Net Amounts of (Liabilities) Presented in the Balance Sheet		(955)
Commodity | Swaps | NGLs (C3-Propane)
Derivative [Line Items]
Gross Amounts of Recognized Assets	0
Gross Amounts Offset in the Balance Sheet	(12,556)
Net Amounts of Assets Presented in the Balance Sheet	(12,556)
Gross Amounts of Recognized (Liabilities)	(34,325)	(32,437)
Gross Amounts Offset in the Balance Sheet	12,556	0
Net Amounts of (Liabilities) Presented in the Balance Sheet	(21,769)	(32,437)
Commodity | Swaps | NGLs (NC4-Normal Butane)
Derivative [Line Items]
Gross Amounts of Recognized Assets	0	4
Gross Amounts Offset in the Balance Sheet	(6,051)	(4)
Net Amounts of Assets Presented in the Balance Sheet	(6,051)	0
Gross Amounts of Recognized (Liabilities)	(11,188)	(13,419)
Gross Amounts Offset in the Balance Sheet	6,051	4
Net Amounts of (Liabilities) Presented in the Balance Sheet	(5,137)	(13,415)
Commodity | Swaps | NGLs (C5-Natural Gasoline)
Derivative [Line Items]
Gross Amounts of Recognized Assets	0
Gross Amounts Offset in the Balance Sheet	(6,727)
Net Amounts of Assets Presented in the Balance Sheet	(6,727)
Gross Amounts of Recognized (Liabilities)	(13,488)	(12,176)
Gross Amounts Offset in the Balance Sheet	6,727	0
Net Amounts of (Liabilities) Presented in the Balance Sheet	(6,761)	(12,176)
Commodity | Swaps | Freight
Derivative [Line Items]
Gross Amounts of Recognized Assets	276	65
Gross Amounts Offset in the Balance Sheet	(276)	(65)
Net Amounts of Assets Presented in the Balance Sheet	0	0
Gross Amounts of Recognized (Liabilities)	0	0
Gross Amounts Offset in the Balance Sheet	276	65
Net Amounts of (Liabilities) Presented in the Balance Sheet	276	65
Commodity | Swaptions | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized Assets	18,817
Gross Amounts Offset in the Balance Sheet	(8,103)
Net Amounts of Assets Presented in the Balance Sheet	10,714
Gross Amounts of Recognized (Liabilities)	(12,283)
Gross Amounts Offset in the Balance Sheet	8,103
Net Amounts of (Liabilities) Presented in the Balance Sheet	(4,180)
Commodity | Basis Swaps | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized Assets	1,815	12,535
Gross Amounts Offset in the Balance Sheet	(6,673)	(9,437)
Net Amounts of Assets Presented in the Balance Sheet	(4,858)	3,098
Gross Amounts of Recognized (Liabilities)	(9,580)	(687)
Gross Amounts Offset in the Balance Sheet	6,673	9,437
Net Amounts of (Liabilities) Presented in the Balance Sheet	(2,907)	8,750
Commodity | Collars | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized Assets	3,039	6,298
Gross Amounts Offset in the Balance Sheet	(500)	(6,298)
Net Amounts of Assets Presented in the Balance Sheet	2,539	0
Gross Amounts of Recognized (Liabilities)	0	(2,625)
Gross Amounts Offset in the Balance Sheet	500	6,298
Net Amounts of (Liabilities) Presented in the Balance Sheet	500	3,673
Commodity | Collars | NGLs (C3-Propane)
Derivative [Line Items]
Gross Amounts of Recognized Assets	85
Gross Amounts Offset in the Balance Sheet	(85)
Net Amounts of Assets Presented in the Balance Sheet	0
Gross Amounts of Recognized (Liabilities)	0
Gross Amounts Offset in the Balance Sheet	85
Net Amounts of (Liabilities) Presented in the Balance Sheet	85
Commodity | Spread Swaps | NGLs (C3-Propane)
Derivative [Line Items]
Gross Amounts of Recognized Assets	12,762	17,396
Gross Amounts Offset in the Balance Sheet	(12,762)	(17,396)
Net Amounts of Assets Presented in the Balance Sheet	0	0
Gross Amounts of Recognized (Liabilities)	(13,983)	(18,138)
Gross Amounts Offset in the Balance Sheet	12,762	17,396
Net Amounts of (Liabilities) Presented in the Balance Sheet	$ (1,221)	(742)
Commodity | Puts | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized Assets		18,159
Gross Amounts Offset in the Balance Sheet		(15,429)
Net Amounts of Assets Presented in the Balance Sheet		2,730
Gross Amounts of Recognized (Liabilities)		0
Gross Amounts Offset in the Balance Sheet		15,429
Net Amounts of (Liabilities) Presented in the Balance Sheet		15,429
Commodity | Calls | Natural Gas
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)		(1,041)
Gross Amounts Offset in the Balance Sheet		0
Net Amounts of (Liabilities) Presented in the Balance Sheet		$ (1,041)