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ASSET RETIREMENT OBLIGATION - Summary of Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
ASSET RETIREMENT OBLIGATION          
Balance, beginning of period     $ 6,203 $ 5,731 $ 5,731
Liabilities settled     (320)   (521)
Liabilities disposed     (13)    
Accretion expense $ 30 $ 134 156 $ 268 993
Balance, end of period 6,026   6,026   6,203
Less: Current portion (881)   (881)   (708)
Non-current Portion $ 5,145   $ 5,145   $ 5,495