Percentage Of Preferred Interest In Company Held By Principal Customer And Related Party

No definition available.

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Namespace Prefix:	mp_
Data Type:	dtr-types:percentItemType
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- Definition

Related Party Transaction, Advances From Related Party

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Implied Discount, Percentage Of Contractual Gross Profit

+ References

No definition available.

+ Details

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

- Definition

Related Party Transaction, Increase In Advances From Related Party

+ References

No definition available.

+ Details

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

- Definition

Related Party Transactions. Extension Period

+ References

No definition available.

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Namespace Prefix:	mp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Related Party Transactions, Initial Term

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No definition available.

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Name:	mp_RelatedPartyTransactionsInitialTerm
Namespace Prefix:	mp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of preferred units of ownership outstanding of a limited liability company (LLC).

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No definition available.

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Name:	us-gaap_LimitedLiabilityCompanyLLCPreferredUnitOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- 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

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

- Details

Name:	dei_LegalEntityAxis=mp_MPMineOperationsLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=mp_OfftakeAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=mp_OriginalOfftakeAgreementInitialPrepaymentAmountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=mp_OriginalOfftakeAgreementFirstAdditionalAdvanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=mp_ShengheImpliedDiscountMember
Namespace Prefix:
Data Type:	na
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Period Type:

RELATIONSHIP AND AGREEMENTS WITH SHENGHE - Offtake Agreement and Original Commercial Agreements (Details) - USD ($) $ in Millions			1 Months Ended		18 Months Ended
	Jun. 05, 2020	Jun. 20, 2017	Mar. 31, 2022	May 31, 2017	Jun. 30, 2020	Nov. 16, 2020	Dec. 31, 2019	May 22, 2017
Related Party Transaction [Line Items]
Preferred interest percentage held by Shenghe								9.99%
MPMO
Related Party Transaction [Line Items]
Preferred units, outstanding (shares)						200.86	110.98
Offtake Agreement \| Affiliated Entity
Related Party Transaction [Line Items]
Initial term			2 years
Extension period			1 year
Initial Prepayment Amount \| Affiliated Entity
Related Party Transaction [Line Items]
Advances				$ 50.0
Increase in advances		$ 30.0
Initial Prepayment Amount \| Affiliated Entity \| Shenghe
Related Party Transaction [Line Items]
Increase in advances	$ 30.0
First Additional Advance \| Affiliated Entity
Related Party Transaction [Line Items]
Advances		$ 30.0
Shenghe Implied Discount \| Affiliated Entity \| Shenghe
Related Party Transaction [Line Items]
Shenghe Implied Discount, percentage of contractual gross profit		36.00%			36.00%