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Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on 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]    
Short-term marketable securities $ 208,199 $ 145,246
Fair Value, Measurements, Recurring Basis    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 306,892 31,555
Short-term marketable securities 208,199 145,246
Total cash equivalents and short-term marketable securities 515,091 176,801
Fair Value, Measurements, Recurring Basis | Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 306,892 31,555
Short-term marketable securities 208,199 145,246
Total cash equivalents and short-term marketable securities 515,091 176,801
Fair Value, Measurements, Recurring Basis | Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term marketable securities 0 0
Total cash equivalents and short-term marketable securities 0 0
Fair Value, Measurements, Recurring Basis | Level 3    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Short-term marketable securities 0 0
Total cash equivalents and short-term marketable securities 0 0
Fair Value, Measurements, Recurring Basis | Money Market Funds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 256,957 31,555
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 256,957 31,555
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Fair Value, Measurements, Recurring Basis | Money Market Funds | Level 3    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0 0
Fair Value, Measurements, Recurring Basis | U.S. Treasuries    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 49,935  
Short-term marketable securities 208,199 145,246
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 49,935  
Short-term marketable securities 208,199 145,246
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0  
Short-term marketable securities 0 0
Fair Value, Measurements, Recurring Basis | U.S. Treasuries | Level 3    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash equivalents 0  
Short-term marketable securities $ 0 $ 0