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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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Other Commitments [Line Items] |  |  |
Asset retirement obligation | $ 19,046 | $ 18,280 | $ 17,033
Canyon Project |  |  |
Other Commitments [Line Items] |  |  |
Loss contingency, estimate of possible loss | 21,590 | 22,130 |
Asset retirement obligation | $ 43,310 | $ 43,460 |

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