v3.26.3
STOCK-BASED COMPENSATION - Schedule of Stock-Based Compensation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 11,446 $ 7,686 $ 35,663 $ 20,591
Income Statement Location [Axis]: us-gaap:CostOfGoodsAndServiceExcludingDepreciationDepletionAndAmortization        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 1,555 2,312 5,909 5,060
Income Statement Location [Axis]: us-gaap:SellingGeneralAndAdministrativeExpense        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 8,999 5,112 27,569 14,813
Income Statement Location [Axis]: mp:AdvancedProjectsAndDevelopmentCosts        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense 227 36 358 161
Income Statement Location [Axis]: mp:StartUpCosts        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total stock-based compensation expense $ 665 $ 226 $ 1,827 $ 557