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Fair Value Measurement - Summary of Fair Value Measurements of Cash Equivalents, and Available-for-Sale Marketable Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities $ 419,272 $ 194,494
Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents 18 24,290
Commercial paper securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents   4,595
Total marketable securities 317,119 110,247
Corporate debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities 83,579 75,755
U.S. government sponsored entity debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities 18,574 8,492
Fair value on recurring basis [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents 18 28,885
Total marketable securities 419,272 194,494
Total cash equivalents and marketable securities 419,290 223,379
Fair value on recurring basis [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents 18 24,290
Fair value on recurring basis [Member] | Commercial paper securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents   4,595
Total marketable securities 317,119 110,247
Fair value on recurring basis [Member] | Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents 18 24,290
Total cash equivalents and marketable securities 18 24,290
Fair value on recurring basis [Member] | Level 1 [Member] | Money market funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents 18 24,290
Fair value on recurring basis [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents   4,595
Total marketable securities 419,272 194,494
Total cash equivalents and marketable securities 419,272 199,089
Fair value on recurring basis [Member] | Level 2 [Member] | Commercial paper securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total cash equivalents   4,595
Total marketable securities 317,119 110,247
Fair value on recurring basis [Member] | Corporate debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities 83,579 75,755
Fair value on recurring basis [Member] | Corporate debt securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities 83,579 75,755
Fair value on recurring basis [Member] | U.S. government sponsored entity debt securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities 18,574 8,492
Fair value on recurring basis [Member] | U.S. government sponsored entity debt securities [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total marketable securities $ 18,574 $ 8,492