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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Other Commitments [Line Items] |  |  |
Asset retirement obligation | $ 19,450 | $ 18,972 | $ 19,104
Canyon Project |  |  |
Other Commitments [Line Items] |  |  |
Loss contingency, estimate of possible loss | 20,130 | 20,080 |
Asset retirement obligation | $ 41,750 | $ 41,750 |

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