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Asset Retirement Obligations - Changes in Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Asset Retirement Obligation [Roll Forward]          
Asset retirement obligations - beginning of period     $ 11,331 $ 12,679 $ 12,679
Liabilities incurred for new wells and interest     636   747
Liabilities settled upon plugging and abandoning wells     (17)   0
Liabilities removed upon sale of wells     0   (2,827)
Revision of estimates     (75)   155
Accretion expense $ 136 $ 159 244 $ 316 577
Asset retirement obligations - end of period $ 12,119   $ 12,119   $ 11,331