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INCOME AND MINING TAXES - Changes to Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation allowance
Balance at Beginning of Period	$ 121,212	$ 124,153	$ 123,648
Additions	39,794	2,104	12,232
Deductions	(6,708)	(5,045)	(11,727)
Balance at End of Period	$ 154,298	$ 121,212	$ 124,153

X

- Definition

Amount of decrease in the valuation allowance for a specified deferred tax asset.

+ References

No definition available.

+ Details

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Namespace Prefix:	mux_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Amount of increase in the valuation allowance for a specified deferred tax asset.

+ References

No definition available.

+ Details

Name:	mux_ValuationAllowanceDeferredTaxAssetIncreaseAmount
Namespace Prefix:	mux_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

Name:	us-gaap_ValuationAllowanceAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration