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Fair Value Measurement - Schedule of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 6,784  
Marketable securities, fair value 521,713  
Recurring Basis [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Other non-current assets 600 $ 600
Total 1,690,933 944,371
Recurring Basis [Member] | Cash [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 395,083 13,998
Recurring Basis [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 742,958 929,773
Recurring Basis [Member] | Corporate Debt Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 2,503  
Marketable securities, fair value 324,456  
Recurring Basis [Member] | Certificates of Deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 12,527  
Marketable securities, fair value 25,162  
Recurring Basis [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 15,549  
Marketable securities, fair value 90,375  
Recurring Basis [Member] | US Treasury Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, fair value 47,979  
Recurring Basis [Member] | Government-sponsored Enterprise Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, fair value 33,741  
Recurring Basis [Member] | Level 1 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 1,138,041 943,771
Recurring Basis [Member] | Level 1 [Member] | Cash [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 395,083 13,998
Recurring Basis [Member] | Level 1 [Member] | Money Market Funds [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 742,958 929,773
Recurring Basis [Member] | Level 2 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total 552,292  
Recurring Basis [Member] | Level 2 [Member] | Corporate Debt Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 2,503  
Marketable securities, fair value 324,456  
Recurring Basis [Member] | Level 2 [Member] | Certificates of Deposit [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 12,527  
Marketable securities, fair value 25,162  
Recurring Basis [Member] | Level 2 [Member] | Commercial Paper [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 15,549  
Marketable securities, fair value 90,375  
Recurring Basis [Member] | Level 2 [Member] | US Treasury Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, fair value 47,979  
Recurring Basis [Member] | Level 2 [Member] | Government-sponsored Enterprise Securities [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, fair value 33,741  
Recurring Basis [Member] | Level 3 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Other non-current assets 600 600
Total $ 600 $ 600