Investments - Summary of Short-term, Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	$ 302,222	$ 224,377
Held to Maturity, Gross Unrealized Gains	1,954	6,164
Held to Maturity, Gross Unrealized Losses	(103)	(79)
Held to Maturity, Fair Value	304,073	230,462
Marketable Securities, Amortized Cost	125,000	85,000
Marketable Securities, Realized Gains/(Losses)	2,481	95
Marketable Securities, Gross Unrealized Gains	135
Marketable Securities, Gross Unrealized Losses	(888)	(75)
Marketable Securities, Fair Value	126,728	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	56,627	86,890
Held to Maturity, Gross Unrealized Gains	803	1,590
Held to Maturity, Fair Value	57,430	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	195,595	137,487
Held to Maturity, Gross Unrealized Gains	1,151	4,574
Held to Maturity, Gross Unrealized Losses	(103)	(79)
Held to Maturity, Fair Value	196,643	141,982
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)	2,481	95
Marketable Securities, Gross Unrealized Gains	135
Marketable Securities, Gross Unrealized Losses	(888)	(75)
Marketable Securities, Fair Value	$ 126,728	$ 85,020

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