Quarterly report pursuant to Section 13 or 15(d)

Open Commodity Derivatives (Detail)

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Open Commodity Derivatives (Detail) (Swaps Oil)
 3 Months Ended
Mar. 31, 2013
2013: 2nd Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
2013: 3rd Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
2013: 4th Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
2014: 1st Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 1st quarter
2014: 2nd Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 2nd quarter
2014: 3rd Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 3rd quarter
2014: 4th Quarter
  
Derivatives, Fair Value [Line Items]  
Termination Period 4th quarter
Ice Brent Crude Oil Futures
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 2,043,800
Weighted Average Contract Price 102.30
Ice Brent Crude Oil Futures | 2013: 2nd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 668,700
Weighted Average Contract Price 106.48
Ice Brent Crude Oil Futures | 2013: 3rd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 405,800
Weighted Average Contract Price 103.85
Ice Brent Crude Oil Futures | 2013: 4th Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 294,400
Weighted Average Contract Price 101.98
Ice Brent Crude Oil Futures | 2014: 1st Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 180,000
Weighted Average Contract Price 97.38
Ice Brent Crude Oil Futures | 2014: 2nd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 172,900
Weighted Average Contract Price 97.38
Ice Brent Crude Oil Futures | 2014: 3rd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 165,600
Weighted Average Contract Price 97.38
Ice Brent Crude Oil Futures | 2014: 4th Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 156,400
Weighted Average Contract Price 97.37
Nymex Crude Oil Futures
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 1,008,000
Weighted Average Contract Price 97.28
Nymex Crude Oil Futures | 2013: 2nd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 244,000
Weighted Average Contract Price 97.25
Nymex Crude Oil Futures | 2013: 3rd Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 185,000
Weighted Average Contract Price 97.13
Nymex Crude Oil Futures | 2013: 4th Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 520,000
Weighted Average Contract Price 97.38
Nymex Crude Oil Futures | 2014: 1st Quarter
  
Derivatives, Fair Value [Line Items]  
Notional Quantity (Bbls) 59,000
Weighted Average Contract Price 97.02