v3.26.3
Compensation Related Costs, Share Based Payments (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]        
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]        
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]        
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]        
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]        
Stock-based compensation expense 665 226 1,827 557
Location, Statement of Financial Position, Activity, Capitalization [Axis]: us-gaap:InventoryNet        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense, amount capitalized 591 0 2,767 0
Location, Statement of Financial Position, Activity, Capitalization [Axis]: us-gaap:PropertyPlantAndEquipmentNet        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense, amount capitalized $ 1,065 $ 1,191 $ 3,521 $ 3,427