Quarterly report pursuant to Section 13 or 15(d)

Derivative Financial Instruments (Estimated Fair Value of Derivative Contracts) (Detail)

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Derivative Financial Instruments (Estimated Fair Value of Derivative Contracts) (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Prepaid and other assets
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of assets commodity derivative contracts $ 9,188 $ 2,341
Other assets
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of assets commodity derivative contracts 4,022 1,746
Accrued liabilities
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of liabilities commodity derivative contracts   $ 7,199