ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%
Asset retirement obligation	$ 20,249	$ 19,104	$ 18,280
Canyon Project
Schedule of Asset Retirement Obligations [Line Items]
Asset retirement obligation	$ 41,730	$ 41,320
Measurement Input, Risk Free Interest Rate | Minimum
Schedule of Asset Retirement Obligations [Line Items]
Fair value assumptions, risk free interest rate	0.095
Measurement Input, Risk Free Interest Rate | Maximum
Schedule of Asset Retirement Obligations [Line Items]
Fair value assumptions, risk free interest rate	0.115