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Fair Value Measurement - Summary of Fair Value Measurements of Cash Equivalents, Available-for-Sale Securities and Free Share Liability (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities $ 303,878 $ 259,715
Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 30,496 103,291
Commercial paper securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 2,999  
Total marketable securities 155,368 177,224
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 95,017 63,870
U.S. government-sponsored entity debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 53,493 18,621
Fair value on recurring basis    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 33,495 103,291
Total marketable securities 303,878 259,715
Total cash equivalents and marketable securities 337,373 363,006
Fair value on recurring basis | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 30,496 103,291
Fair value on recurring basis | Commercial paper securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 2,999  
Total marketable securities 155,368 177,224
Fair value on recurring basis | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 30,496 103,291
Total marketable securities 0 0
Total cash equivalents and marketable securities 30,496 103,291
Fair value on recurring basis | Level 1 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 30,496 103,291
Fair value on recurring basis | Level 1 | Commercial paper securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 0  
Total marketable securities 0 0
Fair value on recurring basis | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 2,999 0
Total marketable securities 303,878 259,715
Total cash equivalents and marketable securities 306,877 259,715
Fair value on recurring basis | Level 2 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 0 0
Fair value on recurring basis | Level 2 | Commercial paper securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 2,999  
Total marketable securities 155,368 177,224
Fair value on recurring basis | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 0 0
Total marketable securities 0 0
Total cash equivalents and marketable securities 0 0
Fair value on recurring basis | Level 3 | Money market funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 0 0
Fair value on recurring basis | Level 3 | Commercial paper securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents 0  
Total marketable securities 0 0
Fair value on recurring basis | Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 95,017 63,870
Fair value on recurring basis | Corporate debt securities | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 0 0
Fair value on recurring basis | Corporate debt securities | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 95,017 63,870
Fair value on recurring basis | Corporate debt securities | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 0 0
Fair value on recurring basis | U.S. government-sponsored entity debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 53,493 18,621
Fair value on recurring basis | U.S. government-sponsored entity debt securities | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 0 0
Fair value on recurring basis | U.S. government-sponsored entity debt securities | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 53,493 18,621
Fair value on recurring basis | U.S. government-sponsored entity debt securities | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total marketable securities 0 0
Fair value on recurring basis | Free shares asset    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents and marketable securities 236  
Fair value on recurring basis | Free shares asset | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents and marketable securities 0  
Fair value on recurring basis | Free shares asset | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents and marketable securities 0  
Fair value on recurring basis | Free shares asset | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total cash equivalents and marketable securities $ 236  
Fair value on recurring basis | Free shares liability    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities   154
Fair value on recurring basis | Free shares liability | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities   0
Fair value on recurring basis | Free shares liability | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities   0
Fair value on recurring basis | Free shares liability | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total liabilities   $ 154