Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Summary of Activity of Asset Retirement Obligations) (Details)

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Asset Retirement Obligations (Summary of Activity of Asset Retirement Obligations) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]        
Asset retirement obligations at beginning of period $ 6,661      
Accretion expense 184 $ 180    
Liabilities incurred 195      
Liabilities settled (88)      
Revisions to estimate (712)      
Asset retirement obligations at end of period $ 6,661   $ 6,240 $ 6,661
Less: Current asset retirement obligations     (1,588) (2,729)
Long-term asset retirement obligations at the end of the period     4,652 3,932
Restricted Investments        
Restricted investments     $ 3,339 $ 3,332