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Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Jun. 30, 2021
Sep. 30, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Short-term investments (held to maturity) $ 195,007 $ 230,461
Fair Value, Measurements, Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 325,981 143,583
Marketable securities 126,407 85,020
Short-term investments (held to maturity) 64,635 88,480
Long-term investments (held to maturity) 130,372 141,981
Fair Value, Measurements, Recurring | Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents 325,981 143,583
Marketable securities 126,407 85,020
Fair Value, Measurements, Recurring | Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Short-term investments (held to maturity) 64,635 88,480
Long-term investments (held to maturity) $ 130,372 $ 141,981