Debt Instrument, Convertible, Business Day Period During Which Debt Can be Converted

No definition available.

Name:	mp_DebtInstrumentConvertibleBusinessDayPeriodDuringWhichDebtCanBeConverted
Namespace Prefix:	mp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Convertible, Percentage of Product Of The Last Reported Sale Price Of the Company’s Common Stock And The Conversion Rate On Each Such Trading Day

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Convertible, Treshold Consecutive Business Days

+ References

No definition available.

+ Details

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

- Definition

Percentage of Par Value of Debt Repurchased

+ References

No definition available.

+ Details

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

- Definition

The price per share of the conversion feature embedded in the debt instrument.

+ References

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount.

+ References

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+ Details

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Balance Type:	na
Period Type:	duration

- Definition

The amount by which the convertible debt's if-converted value exceeds its principle amount at the balance sheet date, regardless of whether the instrument is currently convertible. This element applies to public companies only.

+ References

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Balance Type:	credit
Period Type:	duration

- Definition

The number of equity instruments that the holder of the debt instrument would receive if the debt was converted to equity.

+ References

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Threshold period of specified consecutive trading days within which common stock price to conversion price of convertible debt instrument must exceed threshold percentage for specified number of trading days to trigger conversion feature.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum percentage of common stock price to conversion price of convertible debt instruments to determine eligibility of conversion.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Threshold number of specified trading days that common stock price to conversion price of convertible debt instruments must exceed threshold percentage within a specified consecutive trading period to trigger conversion feature.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

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Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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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.

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+ Details

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- Definition

Percentage price of original principal amount of debt at which debt can be redeemed by the issuer.

+ References

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+ Details

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Period Type:	duration

- Definition

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

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Balance Type:	debit
Period Type:	instant

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow paid to third parties in connection with debt origination, which will be amortized over the remaining maturity period of the associated long-term debt.

+ References

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- Definition

Value of stock issued in lieu of cash for services contributed to the entity. Value of the stock issued includes, but is not limited to, services contributed by vendors and founders.

+ References

No definition available.

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DEBT OBLIGATIONS - Convertible Notes (Details)	1 Months Ended		3 Months Ended		9 Months Ended
DEBT OBLIGATIONS - Convertible Notes (Details)	Mar. 31, 2024 USD ($) d $ / shares	Mar. 31, 2021 USD ($) $ / shares	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)	Mar. 07, 2024 USD ($)
Debt Instrument [Line Items]
Gain on early extinguishment of debt			$ 0	$ 0	$ 46,265,000	$ 0
Debt issuance costs paid in cash					16,149,000	0
Common stock issued in exchange for financial advisory services					$ 3,737,000	$ 0
Convertible Notes due 2026
Debt Instrument [Line Items]
Percentage of par value	$ 0.895
Convertible Notes due 2026 \| Convertible Debt [Member]
Debt Instrument [Line Items]
Amount borrowed		$ 690,000,000
Interest rate		0.25%
Principal amount of debt repurchased	80,000,000						$ 400,000,000
Cash paid to repurchase debt	70,600,000						$ 358,000,000
Debt instrument, convertible, conversion price (in USD per share) \| $ / shares		$ 44.28
Debt instrument, convertible, conversion ratio		0.0225861
Debt instrument, convertible, number of equity instruments (in shares) \| shares					5,999,994
Effective interest rate			0.51%		0.51%
Debt term					1 year 6 months
If-converted value in excess of principal					$ 0
Convertible Notes due 2030 \| Convertible Debt [Member]
Debt Instrument [Line Items]
Amount borrowed	$ 747,500,000
Interest rate	3.00%
Debt instrument, convertible, conversion price (in USD per share) \| $ / shares	$ 21.74
Debt instrument, convertible, conversion ratio	0.0459939
Debt instrument, convertible, number of equity instruments (in shares) \| shares					48,132,646
Effective interest rate			3.49%		3.49%
Debt term					5 years 4 months 24 days
Gross debt issuance costs			$ 19,900,000		$ 19,900,000
Common stock issued in exchange for financial advisory services					3,700,000
Debt instrument, redemption price, percentage	100.00%
If-converted value in excess of principal					$ 102,000,000.0
Convertible Notes due 2030 \| Convertible Debt [Member] \| Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Debt instrument, convertible, threshold trading days \| d	20
Threshold consecutive trading days \| d	30
Debt instrument, convertible, threshold percentage of stock price trigger	130.00%
Debt Instrument, Convertible, Business Day Period During Which Debt Can be Converted \| d	5
Debt Instrument, Convertible, Treshold Consecutive Business Days \| d	10
Debt Instrument, Convertible, Percentage of Product Of The Last Reported Sale Price Of the Company’s Common Stock And The Conversion Rate On Each Such Trading Day	98.00%
Convertible Notes due 2030 \| Convertible Debt [Member] \| Maximum
Debt Instrument [Line Items]
Debt instrument, convertible, conversion ratio	0.0643915