Contract With Customer, Liability, Deferred Revenue, Current

No definition available.

Name:	mp_ContractWithCustomerLiabilityDeferredRevenueCurrent
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contract With Customer, Liability, Related Parties, Deferred Revenue

+ References

No definition available.

+ Details

Name:	mp_ContractWithCustomerLiabilityRelatedPartiesDeferredRevenue
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contract With Customer, Liability, Related Parties, Deferred Revenue, Additions

+ References

No definition available.

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Name:	mp_ContractWithCustomerLiabilityRelatedPartiesDeferredRevenueAdditions
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Contract With Customer, Liability, Related Parties, Derecognition From Contract Modification

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No definition available.

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Name:	mp_ContractWithCustomerLiabilityRelatedPartiesDerecognitionFromContractModification
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract With Customer, Liability, Related Parties, Revenue Recognized

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No definition available.

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Name:	mp_ContractWithCustomerLiabilityRelatedPartiesRevenueRecognized
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contract With Customer, Liability, Related Parties

+ References

No definition available.

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Name:	mp_ContractWithCustomerLiabilityRelatedPartiesRollForward
Namespace Prefix:	mp_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Contract With Customer, Liability, Related Parties, Tariff Rebates

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No definition available.

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Name:	mp_ContractWithCustomerLiabilityRelatedPartiesTariffRebates
Namespace Prefix:	mp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Related Party Transaction, Deferred Revenue Percentage Recognized From Gross Profit

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No definition available.

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Name:	mp_RelatedPartyTransactionDeferredRevenuePercentageRecognizedFromGrossProfit
Namespace Prefix:	mp_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=mp_ShengheResourcesSingaporeInternationalTradingPteLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

REVENUE RECOGNITION - Significant Activity For Deferred Revenue (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Jun. 20, 2017	May 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Contract With Customer, Liability, Related Parties [Rollforward]
Opening balance			$ 35,543
Prepayments received			11,050
Revenue recognized in exchange for debt principal reduction			(9,117)
Effect of June 2020 Modification			(37,476)
Ending balance			$ 0
Deferred revenue expected to be realized, current				$ 6,600
Affiliated Entity \| Shenghe
Contract With Customer, Liability, Related Parties [Rollforward]
Deferred revenue percentage recognized from gross profit	64.00%
Tariff rebates		$ 1,400