Quarterly report pursuant to Section 13 or 15(d)

Open Commodity Derivatives (Detail)

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Open Commodity Derivatives (Detail) (Swaps - Oil, USD $)
 3 Months Ended
Mar. 31, 2014
Ice Brent Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 494,900
Weighted Average Contract Price $ 97.38
Nymex Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 415,000
Weighted Average Contract Price $ 97.04
Argus Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 1,925,000
Weighted Average Contract Price $ 97.84
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) 353,000
Weighted Average Contract Price $ 97.04
2014: 2nd Quarter | Argus Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 637,000
Weighted Average Contract Price $ 97.83
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) 62,000
Weighted Average Contract Price $ 97.01
2014: 3rd Quarter | Argus Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 828,000
Weighted Average Contract Price $ 97.69
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
2014: 4th Quarter | Argus Crude Oil Futures
  
Derivatives Fair Value [Line Items]  
Notional Quantity (Bbls) 460,000
Weighted Average Contract Price $ 98.12