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ASSET RETIREMENT OBLIGATIONS - Retirement obligation rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Changes in the asset retirement obligations
Asset retirement obligation, beginning of the period			$ 32,201	$ 29,402	$ 29,402
Settlements			(185)		(513)
Accretion of liability			1,389		1,680
Adjustment reflecting updated estimates			673		1,012
Foreign exchange revaluation			(286)		620
Asset retirement obligation, ending balance	$ 33,792		33,792		32,201
Less current portion	2,860		2,860		2,610
Long-term portion	30,932		30,932		$ 29,591
Reclamation obligations
Reclamation adjustment reflecting updated estimates	411		614	88
Remediation Accretion	475	$ 495	1,389	1,295
Total	$ 886	$ 495	$ 2,003	$ 1,383

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Amount of reclamation adjustments during the period.

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Amount of reclamation and remediation expenses.

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Amount recognized for the passage of time, typically for liabilities, that have been discounted to their net present values. Excludes accretion associated with asset retirement obligations.

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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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Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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Current portion of 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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Amount of foreign currency translation gain (loss) which decreases (increases) asset retirement obligations.

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Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

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Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Noncurrent portion of 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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