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Property, Plant and Equipment (Narrative) (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Property, Plant and Equipment [Line Items]        
Impairment of oil and gas properties $ 36 $ 396 $ 2,607 $ 3,313
Unproved Oil And Gas Properties [Member]
       
Property, Plant and Equipment [Line Items]        
Impairment of oil and gas properties $ 36 $ 396 $ 2,607 $ 3,313