Quarterly report pursuant to Section 13 or 15(d)

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

v2.4.0.6
Derivative Financial Instruments (Estimated Fair Value of Derivative Contracts) (Detail) (USD $)
In Thousands, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Prepaid And Other Assets [Member]
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of assets commodity derivative contracts   $ 2,341
Other Assets [Member]
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of assets commodity derivative contracts   1,746
Accrued Liabilities [Member]
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of liabilities commodity derivative contracts 6,950 7,199
Other Liabilities [Member]
   
Derivatives, Fair Value [Line Items]    
Estimated fair value of liabilities commodity derivative contracts $ 3,623