Quarterly report pursuant to Section 13 or 15(d)

Open Commodity Derivatives (Detail)

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Open Commodity Derivatives (Detail) (Swaps Oil)
 9 Months Ended
Sep. 30, 2013
Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 969,300
Weighted Average Contract Price 98.77
Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 1,892,000
Weighted Average Contract Price 97.30
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) 762,000
Weighted Average Contract Price 97.39
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: 2nd Quarter | Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 455,000
Weighted Average Contract Price 97.17
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: 3rd Quarter | Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 155,000
Weighted Average Contract Price 97.00
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