Derivative Activities - Schedule of Additional Information Related to Master Netting Arrangements with Derivative Counterparties (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	$ (8,114)	$ (19,743)
Gross Amounts Offset in the Balance Sheet	8,114	11,809
Net Amounts of (Liabilities) Presented in the Balance Sheet		(7,934)
Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	135,633	165,076
Gross Amounts Offset in the Balance Sheet	(8,114)	(11,809)
Net Amounts of Assets Presented in the Balance Sheet	127,519	153,267
Natural gas | Swap | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(3,638)	(3,242)
Gross Amounts Offset in the Balance Sheet	3,638	(221)
Net Amounts of (Liabilities) Presented in the Balance Sheet		(3,463)
Natural gas | Swap | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	14,255	10,746
Gross Amounts Offset in the Balance Sheet	(3,638)	(3,242)
Net Amounts of Assets Presented in the Balance Sheet	10,617	7,504
Natural gas | Collars | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(3,703)	(9,618)
Gross Amounts Offset in the Balance Sheet	3,703	9,618
Net Amounts of (Liabilities) Presented in the Balance Sheet
Natural gas | Collars | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	88,087	128,410
Gross Amounts Offset in the Balance Sheet	(3,703)	(6,155)
Net Amounts of Assets Presented in the Balance Sheet	84,384	122,255
Natural gas | Basis Swap | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets		993
Net Amounts of Assets Presented in the Balance Sheet		993
Crude oil | Swap | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	15,980	9,650
Net Amounts of Assets Presented in the Balance Sheet	15,980	9,650
Crude oil | Collars | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	1,979	2,222
Net Amounts of Assets Presented in the Balance Sheet	1,979	2,222
Swap NGLs (Natural Gasoline) | C5 swaps | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)		(137)
Gross Amounts Offset in the Balance Sheet		2,412
Net Amounts of (Liabilities) Presented in the Balance Sheet		2,275
Swap NGLs (Natural Gasoline) | C5 swaps | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	11,743	13,055
Gross Amounts Offset in the Balance Sheet		(2,412)
Net Amounts of Assets Presented in the Balance Sheet	11,743	10,643
Swap NGLs (Natural Gasoline) | C3 swaps | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(773)	(6,746)
Gross Amounts Offset in the Balance Sheet	773
Net Amounts of (Liabilities) Presented in the Balance Sheet		(6,746)
Swap NGLs (Natural Gasoline) | C3 swaps | Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	3,589
Gross Amounts Offset in the Balance Sheet	(773)
Net Amounts of Assets Presented in the Balance Sheet	$ 2,816