Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Details)

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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations at January 1, 2018 $ 6,020    
Accretion expense 218 $ 184  
Liabilities incurred 44    
Liabilities settled (95)    
Revisions to estimate (a) 4,314    
Asset retirement obligations at end of period 10,501    
Less: Current asset retirement obligations (2,784)   $ (1,295)
Long-term asset retirement obligations at March 31, 2018 7,717   4,725
Restricted Investments      
Restricted investments $ 3,382   $ 3,372