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Fair Value Measurement - Assets and Liabilities Measured at Fair Value (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Assets:      
Money market funds $ 261,585 $ 187,509 $ 470,319
Marketable securities: 32,102 37,827  
Liabilities:      
Deferred payments 100,119 104,742  
Recurring      
Assets:      
Total assets at fair value on a recurring basis 33,985 40,668  
Money market funds | Recurring      
Assets:      
Money market funds 1,883 2,841  
Tax-exempt municipal securities      
Assets:      
Marketable securities: 18,911 20,289  
Tax-exempt municipal securities | Recurring      
Assets:      
Marketable securities: 18,911 20,289  
Corporate debt securities      
Assets:      
Marketable securities: 13,191 17,538  
Corporate debt securities | Recurring      
Assets:      
Marketable securities: 13,191 17,538  
Deferred payments | Recurring      
Liabilities:      
Deferred payments 1,368 1,658  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Recurring      
Assets:      
Total assets at fair value on a recurring basis 1,883 2,841  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Money market funds | Recurring      
Assets:      
Money market funds 1,883 2,841  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Tax-exempt municipal securities | Recurring      
Assets:      
Marketable securities: 0 0  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Corporate debt securities | Recurring      
Assets:      
Marketable securities: 0 0  
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) | Deferred payments | Recurring      
Liabilities:      
Deferred payments 0 0  
Significant Other Observable Inputs (Level 2) | Recurring      
Assets:      
Total assets at fair value on a recurring basis 32,102 37,827  
Significant Other Observable Inputs (Level 2) | Money market funds | Recurring      
Assets:      
Money market funds 0 0  
Significant Other Observable Inputs (Level 2) | Tax-exempt municipal securities | Recurring      
Assets:      
Marketable securities: 18,911 20,289  
Significant Other Observable Inputs (Level 2) | Corporate debt securities | Recurring      
Assets:      
Marketable securities: 13,191 17,538  
Significant Other Observable Inputs (Level 2) | Deferred payments | Recurring      
Liabilities:      
Deferred payments 0 0  
Significant Unobservable Inputs (Level 3) | Recurring      
Assets:      
Total assets at fair value on a recurring basis 0 0  
Significant Unobservable Inputs (Level 3) | Money market funds | Recurring      
Assets:      
Money market funds 0 0  
Significant Unobservable Inputs (Level 3) | Tax-exempt municipal securities | Recurring      
Assets:      
Marketable securities: 0 0  
Significant Unobservable Inputs (Level 3) | Corporate debt securities | Recurring      
Assets:      
Marketable securities: 0 0  
Significant Unobservable Inputs (Level 3) | Deferred payments | Recurring      
Liabilities:      
Deferred payments $ 1,368 $ 1,658