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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Changes in the Company's asset retirement obligations      
Beginning asset retirement obligations $ 26,256    
Liabilities incurred during period 747    
Liabilities settled during period (411)    
Accretion expense 364 $ 300  
Ending asset retirement obligations 26,956    
Less: current asset retirement obligations [1] (1,274)    
Long-term asset retirement obligations $ 25,682   $ 25,080
[1] Included in accrued liabilities in the Company’s interim unaudited condensed consolidated balance sheet at March 31, 2018.