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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Changes in the Company's asset retirement obligations      
Beginning asset retirement obligations $ 20,640    
Liabilities incurred during period 577    
Liabilities settled during period (130)    
Revisions in estimated cash flows 794    
Accretion expense 300 $ 264  
Ending asset retirement obligations 22,181    
Less: current asset retirement obligations [1] (699)    
Long-term asset retirement obligations $ 21,482   $ 19,725
[1] Included in accrued liabilities in the Company’s interim unaudited condensed consolidated balance sheet at March 31, 2017.