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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Changes in the Company's asset retirement obligations          
Beginning asset retirement obligations     $ 5,769    
Liabilities incurred during period     417    
Liabilities settled during period     (57)    
Revisions in estimated cash flows     533    
Accretion expense 86 59 248 170  
Ending asset retirement obligations 6,910   6,910    
Less: current asset retirement obligations (763) [1]   (763) [1]    
Long-term asset retirement obligations $ 6,147   $ 6,147   $ 5,109
[1] Included in accrued liabilities in the Company’s unaudited condensed consolidated balance sheet at September 30, 2013.