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
6 Months Ended
Jun. 30, 2012
bbl
Zero Cost Collars Oil
 
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 375,000
Weighted Average Floor Contract Price 75.00
Weighted Average Ceiling Contract Price 98.62
Fair Value Liability $ 74
Zero Cost Collars Oil | 2012: 3rd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 124,000
Weighted Average Floor Contract Price 75.00
Weighted Average Ceiling Contract Price 97.88
Fair Value Liability 6
Zero Cost Collars Oil | 2012: 4th quarter
 
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 68
Swaps Oil
 
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 2,641,400
Weighted Average Contract Price 102.15
Fair Value Asset 13,136
Swaps Oil | 2012: 3rd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 414,000
Weighted Average Contract Price 107.28
Fair Value Asset 3,860
Swaps Oil | 2012: 4th quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 257,600
Weighted Average Contract Price 107.28
Fair Value Asset 2,405
Swaps Oil | 2013: 1st quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 1st quarter
Notional Quantity (Bbls) 351,000
Weighted Average Contract Price 101.97
Fair Value Asset 1,446
Swaps Oil | 2013: 2nd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
Notional Quantity (Bbls) 336,700
Weighted Average Contract Price 101.97
Fair Value Asset 1,403
Swaps Oil | 2013: 3rd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 312,800
Weighted Average Contract Price 101.98
Fair Value Asset 1,396
Swaps Oil | 2013: 4th quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 294,400
Weighted Average Contract Price 101.98
Fair Value Asset 1,447
Swaps Oil | 2014: 1st quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 1st quarter
Notional Quantity (Bbls) 180,000
Weighted Average Contract Price 97.38
Fair Value Asset 177
Swaps Oil | 2014: 2nd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
Notional Quantity (Bbls) 172,900
Weighted Average Contract Price 97.38
Fair Value Asset 257
Swaps Oil | 2014: 3rd quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
Notional Quantity (Bbls) 165,600
Weighted Average Contract Price 97.38
Fair Value Asset 338
Swaps Oil | 2014: 4th quarter
 
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Notional Quantity (Bbls) 156,400
Weighted Average Contract Price 97.37
Fair Value Asset $ 407