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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Changes in the Company's asset retirement obligations          
Beginning asset retirement obligations     $ 20,640    
Liabilities incurred during period     1,222    
Liabilities settled during period     (176)    
Revisions in estimated cash flows     794    
Accretion expense $ 314 $ 289 614 $ 552  
Ending asset retirement obligations 23,094   23,094    
Less: current asset retirement obligations [1] (703)   (703)    
Long-term asset retirement obligations $ 22,391   $ 22,391   $ 19,725
[1] Included in accrued liabilities in the Company’s interim unaudited condensed consolidated balance sheet at June 30, 2017.