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Asset Retirement Obligation (Details Textual) (USD $)
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2008
Asset Retirement Obligation (Textual) [Abstract]        
Fair value of asset retirement obligations related to the legal requirements       $ 11,500,000
Asset retirement obligation 6,374,000 7,968,000 9,717,000  
Other current liabilities [Member]
       
Asset Retirement Obligation (Textual) [Abstract]        
Asset retirement obligation 3,600,000      
Other long-term liabilities [Member]
       
Asset Retirement Obligation (Textual) [Abstract]        
Asset retirement obligation $ 1,500,000