v3.26.3
FAIR VALUE MEASUREMENTS - Carrying Values and Estimated Fair Values (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Jun. 30, 2026
Dec. 31, 2025
Sep. 30, 2025
Jun. 30, 2025
Dec. 31, 2024
Financial assets:            
Cash and cash equivalents $ 260,336   $ 1,166,011      
Level 1 | Fair Value, Recurring            
Financial assets:            
Cash and cash equivalents 260,336   1,166,011      
Short-term investments 1,077,575   664,275      
Restricted cash 527   1,348      
Derivative instrument 0   0      
Level 1 | Convertible Debt | Convertible Notes due 2026 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments     82,449      
Level 1 | Convertible Debt | Convertible Notes due 2030 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 1,934,645   2,172,782      
Level 1 | Notes Payable, Other Payables | Samarium Project Loan | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Level 1 | Equipment notes | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Level 2 | Fair Value, Recurring            
Financial assets:            
Cash and cash equivalents 0   0      
Short-term investments 0   0      
Restricted cash 0   0      
Derivative instrument 0   0      
Level 2 | Convertible Debt | Convertible Notes due 2026 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments     0      
Level 2 | Convertible Debt | Convertible Notes due 2030 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Level 2 | Notes Payable, Other Payables | Samarium Project Loan | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 91,905   98,081      
Level 2 | Equipment notes | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 30,464   25,339      
Level 3 | Fair Value, Recurring            
Financial assets:            
Cash and cash equivalents 0   0      
Short-term investments 0   0      
Restricted cash 0   0      
Derivative instrument 2,669 $ 11,583 8,708 $ 3,406 $ 4,468 $ 0
Level 3 | Convertible Debt | Convertible Notes due 2026 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments     0      
Level 3 | Convertible Debt | Convertible Notes due 2030 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Level 3 | Notes Payable, Other Payables | Samarium Project Loan | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Level 3 | Equipment notes | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 0   0      
Carrying Amount | Fair Value, Recurring            
Financial assets:            
Cash and cash equivalents 260,336   1,166,011      
Short-term investments 1,077,575   664,275      
Restricted cash 527   1,348      
Derivative instrument 2,669   8,708      
Carrying Amount | Convertible Debt | Convertible Notes due 2026 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments     67,411      
Carrying Amount | Convertible Debt | Convertible Notes due 2030 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 848,263   845,301      
Carrying Amount | Notes Payable, Other Payables | Samarium Project Loan | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 87,990   86,029      
Carrying Amount | Equipment notes | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 30,340   24,270      
Fair Value | Fair Value, Recurring            
Financial assets:            
Cash and cash equivalents 260,336   1,166,011      
Short-term investments 1,077,575   664,275      
Restricted cash 527   1,348      
Derivative instrument 2,669   8,708      
Fair Value | Convertible Debt | Convertible Notes due 2026 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments     82,449      
Fair Value | Convertible Debt | Convertible Notes due 2030 | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 1,934,645   2,172,782      
Fair Value | Notes Payable, Other Payables | Samarium Project Loan | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments 91,905   98,081      
Fair Value | Equipment notes | Fair Value, Nonrecurring            
Financial liabilities:            
Debt Instruments $ 30,464   $ 25,339