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Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	$ 330,109	$ 302,222
Held to Maturity, Gross Unrealized Gains	1,068	1,954
Held to Maturity, Gross Unrealized Losses	(1,374)	(103)
Held to Maturity, Fair Value	329,803	304,073
Marketable Securities, Amortized Cost	125,000	125,000
Marketable Securities, Realized Gains/(Losses)	3,134	2,481
Marketable Securities, Gross Unrealized Gains		135
Marketable Securities, Gross Unrealized Losses	(2,124)	(888)
Marketable Securities, Fair Value	126,010	126,728
Certificate Of Deposit Due Within One Year
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
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	62,537	56,627
Held to Maturity, Gross Unrealized Gains	687	803
Held to Maturity, Fair Value	63,224	57,430
Commercial Note Due Within One Through Three Years
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	217,572	195,595
Held to Maturity, Gross Unrealized Gains	381	1,151
Held to Maturity, Gross Unrealized Losses	(1,374)	(103)
Held to Maturity, Fair Value	216,579	196,643
Marketable Securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Marketable Securities, Amortized Cost	125,000	125,000
Marketable Securities, Realized Gains/(Losses)	3,134	2,481
Marketable Securities, Gross Unrealized Gains		135
Marketable Securities, Gross Unrealized Losses	(2,124)	(888)
Marketable Securities, Fair Value	$ 126,010	$ 126,728

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