Fair Value Measurements - Fair Value Measurements for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Short-term investments (held to maturity)	$ 195,063	$ 230,461
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash and cash equivalents	139,921	143,583
Marketable securities	86,012	85,020
Short-term investments (held to maturity)	80,597	88,480
Long-term investments (held to maturity)	114,466	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	139,921	143,583
Marketable securities	86,012	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)	80,597	88,480
Long-term investments (held to maturity)	$ 114,466	$ 141,981