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Derivative Activities - Schedule of Additional Information Related to Master Netting Arrangements with Derivative Counterparties (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Derivative [Line Items]    
Gross Amounts of Recognized Assets $ 89,550 $ 165,076
Gross Amounts Offset in the Balance Sheet (15,483) (11,809)
Net Amounts of Assets Presented in the Balance Sheet 74,067 153,267
Gross Amounts of Recognized (Liabilities) (23,557) (19,743)
Gross Amounts Offset in the Balance Sheet 15,483 11,809
Net Amounts of (Liabilities) Presented in the Balance Sheet (8,074) (7,934)
Natural gas | Swaps
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 10,965 10,746
Gross Amounts Offset in the Balance Sheet (2,046) (3,242)
Net Amounts of Assets Presented in the Balance Sheet 8,919 7,504
Gross Amounts of Recognized (Liabilities) (1,219) (3,242)
Gross Amounts Offset in the Balance Sheet 2,046 (221)
Net Amounts of (Liabilities) Presented in the Balance Sheet 827 (3,463)
Natural gas | Collars
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 71,699 128,410
Gross Amounts Offset in the Balance Sheet (1,720) (6,155)
Net Amounts of Assets Presented in the Balance Sheet 69,979 122,255
Gross Amounts of Recognized (Liabilities) (1,720) (9,618)
Gross Amounts Offset in the Balance Sheet 1,720 9,618
Natural gas | Basis Swap
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets   993
Net Amounts of Assets Presented in the Balance Sheet   993
Crude oil | Swaps
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 2,962 9,650
Gross Amounts Offset in the Balance Sheet (7,766)  
Net Amounts of Assets Presented in the Balance Sheet (4,804) 9,650
Gross Amounts of Recognized (Liabilities) (7,766)  
Gross Amounts Offset in the Balance Sheet 7,766  
Crude oil | Collars
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 137 2,222
Gross Amounts Offset in the Balance Sheet (1,803)  
Net Amounts of Assets Presented in the Balance Sheet (1,666) 2,222
Gross Amounts of Recognized (Liabilities) (1,803)  
Gross Amounts Offset in the Balance Sheet 1,803  
Swap NGLs (Natural Gasoline) | C5 swaps
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 3,374 13,055
Gross Amounts Offset in the Balance Sheet (1,461) (2,412)
Net Amounts of Assets Presented in the Balance Sheet 1,913 10,643
Gross Amounts of Recognized (Liabilities) (9) (137)
Gross Amounts Offset in the Balance Sheet 1,461 2,412
Net Amounts of (Liabilities) Presented in the Balance Sheet 1,452 2,275
Swap NGLs (Natural Gasoline) | C3 swaps
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 7  
Gross Amounts Offset in the Balance Sheet (552)  
Net Amounts of Assets Presented in the Balance Sheet (545)  
Gross Amounts of Recognized (Liabilities) (10,469) (6,746)
Gross Amounts Offset in the Balance Sheet 552  
Net Amounts of (Liabilities) Presented in the Balance Sheet (9,917) (6,746)
Swap NGLs (Natural Gasoline) | C4 swaps
    
Derivative [Line Items]    
Gross Amounts of Recognized Assets 406  
Gross Amounts Offset in the Balance Sheet (135)  
Net Amounts of Assets Presented in the Balance Sheet 271  
Gross Amounts of Recognized (Liabilities) (571)  
Gross Amounts Offset in the Balance Sheet 135  
Net Amounts of (Liabilities) Presented in the Balance Sheet $ (436)