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Fair Value Measurements - Schedule of Financial Assets Measured on a Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities $ 196,170 $ 300,535
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 69,759 59,741
U.S. Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 29,101 59,087
Foreign Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 10,688 5,046
U.S. Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 32,368 52,800
Fair Value, Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 256,278 332,371
Fair Value, Recurring | Foreign Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 61,248 123,861
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 69,759 59,741
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 33,900 59,087
Fair Value, Recurring | Foreign Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 10,688 5,046
Fair Value, Recurring | U.S. Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Available-for-sale securities 32,368 61,885
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 118,074 82,492
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 69,759 59,741
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 138,204 249,879
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 61,248 123,861
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 33,900 59,087
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 10,688 5,046
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 32,368 61,885
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 48,315 22,751
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 $ 48,315 $ 22,751