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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Changes in the Company's asset retirement obligations      
Beginning asset retirement obligations $ 31,086    
Liabilities incurred during period 445    
Accretion expense 414 $ 364  
Ending asset retirement obligations 31,945    
Less: current asset retirement obligations [1] (1,655)    
Long-term asset retirement obligations $ 30,290   $ 29,736
[1] Included in accrued liabilities in the Company’s interim unaudited condensed consolidated balance sheet at March 31, 2019.