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FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Jan. 31, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Increase in liability as a result of increasing the profit element and risk premium input by 100 basis points $ 300    
Decrease in liability as a result of decreasing the profit element and risk premium input by 100 basis points 300    
Increase in liability as a result of decreasing the discount rate by 100 basis points 1,500    
Decrease in liability as a result of increasing the discount rate by 100 basis points 1,700    
Level 2 | Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt securities 0   $ 0
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2028      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Notes payable fair value 538,300    
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2026      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Notes payable fair value 596,100    
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2025      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Notes payable fair value 218,700    
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2024      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Notes payable fair value 9,100    
Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt securities   $ 25,000  
Level 3 | Recurring      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt securities $ 26,437   $ 0