v3.26.1
SEGMENT REPORTING - Segment Reporting Information (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Apr. 17, 2026
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]          
Depreciation, depletion and amortization   $ (35,379) $ (20,777) $ (67,516) $ (42,161)
Interest expense, net   (9,703) (5,414) (19,549) (13,029)
Share-based compensation expense, employee   (11,287) (5,427) (24,154) (12,780)
Initial start-up costs   (13,588) (634) (18,441) (1,406)
Transaction-related and other costs(6)   285 (5,128) (10,204) (7,944)
Accretion of asset retirement and environmental obligations   (385) (372) (771) (745)
Loss on disposals of long-lived assets, net   (168) 991 (168) 1,607
Other income, net   12,397 6,572 32,723 21,790
Loss before income taxes   (29,335) (42,724) (42,977) (69,899)
Additions of property, plant and equipment   230,335 29,006 307,711 59,473
Stock-based compensation expense   11,287 5,552 24,217 12,905
Litigation settlement interest $ 8,800        
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization          
Segment Reporting [Line Items]          
Stock-based compensation expense   1,804 714 4,354 2,748
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense          
Segment Reporting [Line Items]          
Stock-based compensation expense   8,965 4,586 18,570 9,701
Income Statement Location [Axis]: mp:AdvancedProjectsAndDevelopmentCosts          
Segment Reporting [Line Items]          
Stock-based compensation expense   43 125 131 125
Income Statement Location [Axis]: mp:StartUpCosts          
Segment Reporting [Line Items]          
Stock-based compensation expense   475 127 1,162 331
Segment Reporting, Reconciling Item, Corporate Nonsegment          
Segment Reporting [Line Items]          
Corporate expenses and other   (10,983) (7,946) (20,570) (14,893)
Segment capital expenditures   305 8 524 8
Operating Segments          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (40,037) 4,589 (86,361) 338
Segment capital expenditures   230,030 28,998 307,187 59,465
Operating Segments | Materials Segment          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (32,505) 12,678 (69,237) 8,920
Segment capital expenditures   85,773 12,581 116,319 27,924
Operating Segments | Magnetics Segment          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (7,532) (8,089) (17,124) (8,582)
Segment capital expenditures   144,257 $ 16,417 190,868 $ 31,541
Intersegment Eliminations          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   $ (561)   $ (688)