Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations (Details)

v3.8.0.1
Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligations at beginning of period $ 6,661 $ 5,107      
Accretion expense 677 958 $ 660    
Liabilities incurred 278 84      
Liabilities settled (711) (2,378)      
Revisions to estimate (885) 2,890      
Asset retirement obligations at end of period $ 6,661 $ 5,107 $ 5,107 $ 6,020 $ 6,661
Less: current asset retirement obligations       (1,295) (2,729)
Long-term asset retirement obligations at the end of the period       4,725 3,932
Restricted Investments          
Restricted investments       $ 3,372 $ 3,332