Quarterly report pursuant to Section 13 or 15(d)

Open Commodity Derivatives (Detail)

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Open Commodity Derivatives (Detail) (Swaps Oil)
 6 Months Ended
Jun. 30, 2013
Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 1,375,100
Weighted Average Contract Price 100.27
Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 764,000
Weighted Average Contract Price 97.29
2013: 3rd Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 3rd quarter
2013: 3rd Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 405,800
Weighted Average Contract Price 103.85
2013: 3rd Quarter | Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 185,000
Weighted Average Contract Price 97.13
2013: 4th Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 4th quarter
2013: 4th Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 294,400
Weighted Average Contract Price 101.98
2013: 4th Quarter | Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 520,000
Weighted Average Contract Price 97.38
2014: 1st Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 1st quarter
2014: 1st Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 180,000
Weighted Average Contract Price 97.38
2014: 1st Quarter | Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 59,000
Weighted Average Contract Price 97.02
2014: 2nd Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 2nd quarter
2014: 2nd Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 172,900
Weighted Average Contract Price 97.38
2014: 3rd Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 3rd quarter
2014: 3rd Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 165,600
Weighted Average Contract Price 97.38
2014: 4th Quarter
  
Derivatives Fair Value [Line Items]  
Termination Period 4th quarter
2014: 4th Quarter | Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 156,400
Weighted Average Contract Price 97.37