Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	$ 190,249	$ 224,377
Held to Maturity, Gross Unrealized Gains	4,837	6,163
Held to Maturity, Gross Unrealized Losses	(23)	(79)
Held to Maturity, Fair Value	195,063	230,461
Marketable Securities, Amortized Cost	85,095	85,000
Marketable Securities, Realized Gains/(Losses)	477	95
Marketable Securities, Gross Unrealized Gains	440
Marketable Securities, Gross Unrealized Losses		(75)
Marketable Securities, Fair Value	86,012	85,020
Commercial Notes Due Within One Year
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	79,394	86,890
Held to Maturity, Gross Unrealized Gains	1,203	1,590
Held to Maturity, Fair Value	80,597	88,480
Commercial Notes Due Within One Through Three Years
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	110,855	137,487
Held to Maturity, Gross Unrealized Gains	3,634	4,573
Held to Maturity, Gross Unrealized Losses	(23)	(79)
Held to Maturity, Fair Value	114,466	141,981
Marketable Securities
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Marketable Securities, Amortized Cost	85,095	85,000
Marketable Securities, Realized Gains/(Losses)	477	95
Marketable Securities, Gross Unrealized Gains	440
Marketable Securities, Gross Unrealized Losses		(75)
Marketable Securities, Fair Value	$ 86,012	$ 85,020

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Schedule of held to maturity securities and marketable securities.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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