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Fair Value Measurements - Schedule of Financial Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities $ 163,005 $ 319,074
U.S. Treasury Bills and Government Agency Obligations    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 70,491 140,626
U.S. Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 25,734 80,170
Foreign Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 23,648 23,203
U.S. Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 14,995 32,801
Fair Value, Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 258,146 390,972
Fair Value, Recurring | Foreign Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 35,134 57,241
Fair Value, Recurring | U.S. Treasury Bills and Government Agency Obligations    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 70,491 140,626
Fair Value, Recurring | U.S. Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 25,734 80,170
Fair Value, Recurring | Foreign Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 23,648 23,203
Fair Value, Recurring | U.S. Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 14,995 32,801
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 158,635 197,557
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury Bills and Government Agency Obligations    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 70,491 140,626
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 99,511 193,415
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Foreign Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 35,134 57,241
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 25,734 80,170
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Foreign Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 23,648 23,203
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 14,995 32,801
Fair Value, Recurring | Cash and Money Market Funds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and money market funds 88,144 56,931
Fair Value, Recurring | Cash and Money Market Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and money market funds $ 88,144 $ 56,931