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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
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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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