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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
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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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