Quarterly report pursuant to Section 13 or 15(d)

Outstanding Derivatives on Individual Contract Basis (Detail)

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Outstanding Derivatives on Individual Contract Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2011
Derivative Instruments and Hedging Activities Disclosure [Line Items]      
Fair Value of Derivatives with Unrealized Gains $ 57,135 $ 46,328 $ 23,839
Fair Value of Derivatives with Unrealized Losses 21,618 23,513 63,906
Designated as Hedging Instrument
      
Derivative Instruments and Hedging Activities Disclosure [Line Items]      
Fair Value of Derivatives with Unrealized Gains 56,894 45,071 22,141
Fair Value of Derivatives with Unrealized Losses 18,977 22,406 63,722
Dedesignated As Hedging Instrument
      
Derivative Instruments and Hedging Activities Disclosure [Line Items]      
Fair Value of Derivatives with Unrealized Gains 170 1,245 1,698
Fair Value of Derivatives with Unrealized Losses 2,427 930 184
Not Designated as Hedging Instrument
      
Derivative Instruments and Hedging Activities Disclosure [Line Items]      
Fair Value of Derivatives with Unrealized Gains 71 12  
Fair Value of Derivatives with Unrealized Losses $ 214 $ 177