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Property, Plant And Equipment (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Property, Plant and Equipment [Line Items]      
Asset retirement costs $ 11,400,000 $ 6,300,000  
US Results of Operations, Impairment of Oil and Gas Properties 3,610,000 11,967,000 6,233,000
Proved Oil And Gas Properties [Member]
     
Property, Plant and Equipment [Line Items]      
US Results of Operations, Impairment of Oil and Gas Properties 0 0 800,000
Unproved Oil And Gas Properties [Member]
     
Property, Plant and Equipment [Line Items]      
US Results of Operations, Impairment of Oil and Gas Properties $ 3,600,000 $ 12,000,000 $ 5,400,000