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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Changes in the Company's asset retirement obligations          
Beginning asset retirement obligations     $ 20,640    
Liabilities incurred during period     1,901    
Liabilities settled during period     (349)    
Revisions in estimated cash flows     794    
Accretion expense $ 323 $ 276 937 $ 828  
Ending asset retirement obligations 23,923   23,923    
Less: current asset retirement obligations [1] (618)   (618)    
Long-term asset retirement obligations $ 23,305   $ 23,305   $ 19,725
[1] Included in accrued liabilities in the Company’s interim unaudited condensed consolidated balance sheet at September 30, 2017.