Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	$ 192,300	$ 224,377
Held to Maturity, Gross Unrealized Gains	2,707	6,163
Held to Maturity, Gross Unrealized Losses		(79)
Held to Maturity, Fair Value	195,007	230,461
Marketable Securities, Amortized Cost	125,000	85,000
Marketable Securities, Realized Gains/(Losses)	1,796	95
Marketable Securities, Gross Unrealized Gains	204
Marketable Securities, Gross Unrealized Losses	(593)	(75)
Marketable Securities, Fair Value	126,407	85,020
Certificate Of Deposit Due Within Two Years
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	50,000
Held to Maturity, Fair Value	50,000
Commercial Note Due Within One Year
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	63,924	86,890
Held to Maturity, Gross Unrealized Gains	711	1,590
Held to Maturity, Fair Value	64,635	88,480
Commercial Note Due Within One Through Three Years
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	78,376	137,487
Held to Maturity, Gross Unrealized Gains	1,996	4,573
Held to Maturity, Gross Unrealized Losses		(79)
Held to Maturity, Fair Value	80,372	141,981
Marketable Securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Marketable Securities, Amortized Cost	125,000	85,000
Marketable Securities, Realized Gains/(Losses)	1,796	95
Marketable Securities, Gross Unrealized Gains	204
Marketable Securities, Gross Unrealized Losses	(593)	(75)
Marketable Securities, Fair Value	$ 126,407	$ 85,020

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