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REVENUE RECOGNITION - Contract Balances (Details) - USD ($) $ in Thousands | 1 Months Ended |  |
--- | --- | --- | ---
Apr. 30, 2024 | Sep. 30, 2024 | Dec. 31, 2023
Revenue from Contract with Customer [Abstract] |  |  |
Deferred revenue additions | $ 50,000 |  |
Deferred revenue |  | $ 50,000 | $ 0

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- Definition Contract With Customer, Liability, Additions + References No definition available. + Details
Name: | mp_ContractWithCustomerLiabilityAdditions
Namespace Prefix: | mp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 606 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479837/606-10-45-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 606 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479806/606-10-50-8 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 606 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479837/606-10-45-2 + Details
Name: | us-gaap_ContractWithCustomerLiabilityCurrent
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_RevenueFromContractWithCustomerAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration