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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%
Asset retirement obligation	$ 14,021	$ 13,038	$ 18,972
Canyon Project
Schedule of Asset Retirement Obligations [Line Items]
Asset retirement obligation	$ 42,290	$ 41,950
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

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[Line Items] for Schedule of Asset Retirement Obligations [Table]

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Value of input used to measure alternative investment.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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