v3.26.1
STOCK-BASED COMPENSATION - Schedule of Stock-Based Compensation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 11,287 $ 5,552 $ 24,217 $ 12,905
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 1,804 714 4,354 2,748
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 8,965 4,586 18,570 9,701
Income Statement Location [Axis]: mp:AdvancedProjectsAndDevelopmentCosts        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 43 125 131 125
Income Statement Location [Axis]: mp:StartUpCosts        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 475 $ 127 $ 1,162 $ 331