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Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	$ 176,638	$ 224,377
Held to Maturity, Gross Unrealized Gains	3,542	6,163
Held to Maturity, Gross Unrealized Losses	(66)	(79)
Held to Maturity, Fair Value	180,114	230,461
Marketable Securities, Amortized Cost	125,000	85,000
Marketable Securities, Realized Gains/(Losses)	1,079	95
Marketable Securities, Gross Unrealized Gains	205
Marketable Securities, Gross Unrealized Losses	(491)	(75)
Marketable Securities, Fair Value	125,793	85,020
Commercial Notes Due Within One Year
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items]
Held to Maturity, Amortized Cost	79,148	86,890
Held to Maturity, Gross Unrealized Gains	887	1,590
Held to Maturity, Fair Value	80,035	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	97,490	137,487
Held to Maturity, Gross Unrealized Gains	2,655	4,573
Held to Maturity, Gross Unrealized Losses	(66)	(79)
Held to Maturity, Fair Value	100,079	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)	1,079	95
Marketable Securities, Gross Unrealized Gains	205
Marketable Securities, Gross Unrealized Losses	(491)	(75)
Marketable Securities, Fair Value	$ 125,793	$ 85,020

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