Annual report pursuant to Section 13 and 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
Dec. 31, 2012
Dec. 31, 2011
Prepaid And Other Assets
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts   $ 2,341
Other Assets
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts   1,746
Accrued Liabilities
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts 6,355 7,199
Other Long-Term Liabilities
   
Derivatives, Fair Value [Line Items]    
Fair value of commodity derivative contracts $ 3,046