Quarterly report pursuant to Section 13 or 15(d)

Estimated Fair Value of Derivative Contracts (Detail)

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Estimated Fair Value of Derivative Contracts (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Prepaid And Other Assets
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts $ 867  
Accrued Liabilities
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts 6,938 6,355
Other Liabilities Noncurrent
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts $ 2,427 $ 3,046