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
Jun. 30, 2013
Dec. 31, 2012
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts $ 2,856  
Fair value of commodity derivative contracts 475  
Prepaid And Other Assets
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts 2,635  
Other Assets
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts 221  
Accrued Liabilities
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts 475 6,355
Other Liabilities Noncurrent
   
Derivatives Fair Value [Line Items]    
Fair value of commodity derivative contracts   $ 3,046