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FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2020
Mar. 09, 2020
Jun. 05, 2019
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 $ 200,000    
Decrease in liability as a result of decreasing the profit element and risk premium input by 100 basis points 200,000    
Decrease in liability as a result of increasing the discount rate by 100 basis points 900,000    
Increase in liability as a result of decreasing the discount rate by 100 basis points 1,000,000.0    
Convertible Notes | Convertible Senior Notes Due 2025      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument face amount   $ 320,000,000.0  
Interest rate   0.25%  
Debt conversion price (in USD per share)   $ 81.54  
Convertible Notes | Convertible Senior Notes Due 2024      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument face amount     $ 132,000,000.0
Interest rate     1.00%
Debt conversion price (in USD per share)     $ 20.5010
Convertible Notes | Minimum | Convertible Senior Notes Due 2025      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt conversion price (in USD per share)   81.54  
Convertible Notes | Maximum | Convertible Senior Notes Due 2025      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt conversion price (in USD per share)   $ 106.94  
Level 1 | Money market funds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Cash and cash equivalents 542,200,000    
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 262,300,000    
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 $ 252,900,000