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INCOME AND MINING TAXES - Changes to Valuation Allowance (Details) - USD ($) $ in Thousands | 12 Months Ended
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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

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- 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

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- 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

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

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- References No definition available. + Details
Name: | us-gaap_ValuationAllowanceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration