v3.26.3
SEGMENT REPORTING - Segment Reporting Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Apr. 17, 2026
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]          
Depreciation, depletion and amortization   $ (36,131) $ (22,497) $ (103,647) $ (64,658)
Interest expense, net   (9,409) (8,566) (28,958) (21,595)
Share-based compensation expense, employee   (11,321) (7,654) (35,475) (20,434)
Initial start-up costs   (12,591) (1,180) (31,032) (2,586)
Transaction-related and other costs(6)   (1,133) (22,364) (11,337) (30,308)
Accretion of asset retirement and environmental obligations   (386) (373) (1,157) (1,118)
Loss on disposals of long-lived assets, net   (1,634) (385) (1,802) 1,222
Other income, net   4,217 17,157 36,940 38,947
Loss before income taxes   (27,250) (58,432) (70,227) (128,331)
Additions of property, plant and equipment   112,539 50,496 420,250 109,969
Stock-based compensation expense   11,446 7,686 35,663 20,591
Litigation settlement interest $ 8,800        
Costs Incurred in Association with the DoW Transactions     10,600   12,400
Nonrefundable Commitment and Structuring Fees Associated with the Commitment Letter     7,400   7,400
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization          
Segment Reporting [Line Items]          
Stock-based compensation expense   1,555 2,312 5,909 5,060
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense          
Segment Reporting [Line Items]          
Stock-based compensation expense   8,999 5,112 27,569 14,813
Income Statement Location [Axis]: mp:AdvancedProjectsAndDevelopmentCosts          
Segment Reporting [Line Items]          
Stock-based compensation expense   227 36 358 161
Income Statement Location [Axis]: mp:StartUpCosts          
Segment Reporting [Line Items]          
Stock-based compensation expense   665 226 1,827 557
Segment Reporting, Reconciling Item, Corporate Nonsegment          
Segment Reporting [Line Items]          
Corporate expenses and other   (11,282) (7,529) (31,852) (22,422)
Segment capital expenditures   310 232 834 240
Operating Segments          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (53,176) 5,041 (139,537) 5,379
Segment capital expenditures   112,229 50,264 419,416 109,729
Operating Segments | Materials Segment          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (47,828) 14,522 (117,065) 23,442
Segment capital expenditures   56,059 31,939 172,378 59,863
Operating Segments | Magnetics Segment          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   (5,348) (9,481) (22,472) (18,063)
Segment capital expenditures   56,170 $ 18,325 247,038 $ 49,866
Intersegment Eliminations          
Segment Reporting [Line Items]          
Elimination of intersegment Adjusted EBITDA   $ (756)   $ (1,444)