Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments (Schedule of Derivative Liabilities at Fair Value) (Detail)

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Derivative Financial Instruments (Schedule of Derivative Liabilities at Fair Value) (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2012
bbl
Swaps Oil [Member]
 
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 2,227,400
Weighted Average Contract Price 101.19
Fair Value Liability $ 10,228
Swaps Oil [Member] | 2012: 4th Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 257,600
Weighted Average Contract Price 107.28
Fair Value Liability 977
Swaps Oil [Member] | 2013: 1st quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 1st quarter
Notional Quantity (Bbls) 351,000
Weighted Average Contract Price 101.97
Fair Value Liability 2,501
Swaps Oil [Member] | 2013: 2nd Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
Notional Quantity (Bbls) 336,700
Weighted Average Contract Price 101.97
Fair Value Liability 1,862
Swaps Oil [Member] | 2013: 3rd Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 312,800
Weighted Average Contract Price 101.98
Fair Value Liability 1,265
Swaps Oil [Member] | 2013: 4th Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 294,400
Weighted Average Contract Price 101.98
Fair Value Liability 805
Swaps Oil [Member] | 2014: 1st Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 1st quarter
Notional Quantity (Bbls) 180,000
Weighted Average Contract Price 97.38
Fair Value Liability 1,073
Swaps Oil [Member] | 2014: 2nd Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
Notional Quantity (Bbls) 172,900
Weighted Average Contract Price 97.38
Fair Value Liability 807
Swaps Oil [Member] | 2014: 3rd Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 165,600
Weighted Average Contract Price 97.38
Fair Value Liability 573
Swaps Oil [Member] | 2014: 4th Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 156,400
Weighted Average Contract Price 97.37
Fair Value Liability 365
Zero Cost Collars Oil [Member] | 2012: 4th Quarter [Member]
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 251,000
Weighted Average Floor Contract Price 75.00
Weighted Average Ceiling Contract Price 98.99
Fair Value Liability $ 345