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COMMITMENTS AND CONTINGENCIES - Streaming Agreement (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Aug. 25, 2017 $ / oz | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($)
Black Fox Complex |  |  |  |  |
Other commitments |  |  |  |  |
Obligation to sell (as a percent) | 8.00% |  |  |  |
Long term gold price (in dollars per ounce) | $ / oz | 561 |  |  |  |
Revenue of acquiree since acquisition date and implementation of streaming agreement | $ |  | $ 0.3 | $ 0.2 | $ 0.9 | $ 0.9
Black Fox Complex | Maximum [Member] |  |  |  |  |
Other commitments |  |  |  |  |
Inflation adjustment (as a percent) | 2.00% |  |  |  |
Pike River property |  |  |  |  |
Other commitments |  |  |  |  |
Obligation to sell (as a percent) | 6.30% |  |  |  |

X
- Definition Represents the long term gold price per ounce used to find the present value of an amount to be paid or received in the future as an input to measure the fair value. + References No definition available. + Details
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Namespace Prefix: | mux_
Data Type: | srt-types:perUnitItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents inflation adjustment of the long term gold price per ounce. + References No definition available. + Details
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Namespace Prefix: | mux_
Data Type: | dtr-types:percentItemType
Balance Type: | na
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- Definition The percentage the Company is obligated to sell as part of the streaming agreement. + References No definition available. + Details
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Namespace Prefix: | mux_
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- Definition This element represents the amount of revenue of the acquiree since the acquisition date and implementation of the streaming agreement. + References No definition available. + Details
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Namespace Prefix: | mux_
Data Type: | xbrli:monetaryItemType
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |