Derivative Activities - Schedule of Additional Information Related to Master Netting Arrangements with Derivative Counterparties (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts Offset in the Balance Sheet	$ 18,132	$ 2,801
Gross Amounts of Recognized (Liabilities)	(46,164)	(10,735)
Net Amounts of (Liabilities) Presented in the Balance Sheet	(28,032)	(7,934)
Natural gas | Swap | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(19,918)
Net Amounts of (Liabilities) Presented in the Balance Sheet	(19,918)
Natural gas | Collars | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts Offset in the Balance Sheet	10,269	420
Gross Amounts of Recognized (Liabilities)	(16,475)	(3,883)
Net Amounts of (Liabilities) Presented in the Balance Sheet	(6,206)	(3,463)
Crude oil | Swap | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts Offset in the Balance Sheet	3,520
Gross Amounts of Recognized (Liabilities)	(690)
Net Amounts of (Liabilities) Presented in the Balance Sheet	2,830
Swap NGLs (Natural Gasoline) | C5 swaps | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts Offset in the Balance Sheet	4,343	2,381
Gross Amounts of Recognized (Liabilities)	(877)	(106)
Net Amounts of (Liabilities) Presented in the Balance Sheet	3,466	2,275
Swap NGLs (Natural Gasoline) | C3 swaps | Derivative Financial Instruments, Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(8,204)	(6,746)
Net Amounts of (Liabilities) Presented in the Balance Sheet	(8,204)	(6,746)
Derivative Financial Instruments, Assets
Derivative [Line Items]
Gross Amounts of Recognized Assets	50,788	162,276
Gross Amounts Offset in the Balance Sheet	(23,349)	(9,009)
Net Amounts of Assets Presented in theBalance Sheet	27,439	153,267
Derivative Financial Instruments, Assets | Natural gas | Swap
Derivative [Line Items]
Gross Amounts of Recognized Assets		10,746
Gross Amounts Offset in the Balance Sheet	(12,701)	(3,242)
Net Amounts of Assets Presented in theBalance Sheet	(12,701)	7,504
Derivative Financial Instruments, Assets | Natural gas | Collars
Derivative [Line Items]
Gross Amounts of Recognized Assets	42,866	127,991
Gross Amounts Offset in the Balance Sheet	(9,314)	(5,736)
Net Amounts of Assets Presented in theBalance Sheet	33,552	122,255
Derivative Financial Instruments, Assets | Natural gas | Basis Swap
Derivative [Line Items]
Gross Amounts of Recognized Assets		993
Net Amounts of Assets Presented in theBalance Sheet		993
Derivative Financial Instruments, Assets | Crude oil | Swap
Derivative [Line Items]
Gross Amounts of Recognized Assets	2,360	9,650
Gross Amounts Offset in the Balance Sheet	(591)
Net Amounts of Assets Presented in theBalance Sheet	1,769	9,650
Derivative Financial Instruments, Assets | Crude oil | Collars
Derivative [Line Items]
Gross Amounts of Recognized Assets	444	2,222
Gross Amounts Offset in the Balance Sheet	(503)
Net Amounts of Assets Presented in theBalance Sheet	(59)	2,222
Derivative Financial Instruments, Assets | Swap NGLs (Natural Gasoline) | C5 swaps
Derivative [Line Items]
Gross Amounts of Recognized Assets	5,084	10,674
Gross Amounts Offset in the Balance Sheet	(240)	(31)
Net Amounts of Assets Presented in theBalance Sheet	4,844	10,643
Derivative Financial Instruments, Assets | Swap NGLs (Natural Gasoline) | C3 swaps
Derivative [Line Items]
Gross Amounts of Recognized Assets	34
Net Amounts of Assets Presented in theBalance Sheet	$ 34