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Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Sep. 30, 2020
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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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