| Document and Entity Information | Sep. 16, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | MP Materials Corp. / DE |
| Amendment Flag | false |
| Entity Central Index Key | 0001801368 |
| Document Type | 8-K |
| Document Period End Date | Sep. 16, 2021 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-39277 |
| Entity Tax Identification Number | 84-4465489 |
| Entity Address, Address Line One | 6720 Via Austi Parkway |
| Entity Address, Address Line Two | Suite 450 |
| Entity Address, City or Town | Las Vegas |
| Entity Address, State or Province | NV |
| Entity Address, Postal Zip Code | 89119 |
| City Area Code | (702) |
| Local Phone Number | 844-6111 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Class A common stock, par value of $0.0001 per share |
| Trading Symbol | MP |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
