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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Assets:    
Marketable debt securities $ 49,995,000 $ 0
Total 751,725,000 314,850,000
Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Total 374,014,000 291,810,000
Significant Other Observable Inputs (Level 2)    
Assets:    
Total 377,711,000 23,040,000
Significant Unobservable Inputs (Level 3)    
Assets:    
Total 0 0
Money market funds    
Assets:    
Cash equivalents 374,014,000 291,810,000
Money market funds | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 374,014,000 291,810,000
Money market funds | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 0 0
Money market funds | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0 0
Treasury discount notes    
Assets:    
Cash equivalents 324,995,000  
Marketable debt securities 49,995,000  
Treasury discount notes | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 0  
Marketable debt securities 0  
Treasury discount notes | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 324,995,000  
Marketable debt securities 49,995,000  
Treasury discount notes | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents 0  
Marketable debt securities 0  
Time deposits    
Assets:    
Cash equivalents 2,721,000 23,040,000
Time deposits | Quoted Market Prices in Active Markets for Identical Assets (Level 1)    
Assets:    
Cash equivalents 0 0
Time deposits | Significant Other Observable Inputs (Level 2)    
Assets:    
Cash equivalents 2,721,000 23,040,000
Time deposits | Significant Unobservable Inputs (Level 3)    
Assets:    
Cash equivalents $ 0 $ 0