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Investments - Summary of Short-term and Long-term Investments and Marketable Securities (Detail) - USD ($) $ in Thousands | Mar. 31, 2022 | Sep. 30, 2021
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Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Short-term and Long-term Investments and Marketable Securities, Cost Basis | $ 549,985 | $ 429,703
Short-term and Long-term Investments and Marketable Securities, Gross Unrealized Gains | 226 | 1,954
Short-term and Long-term Investments and Marketable Securities, Gross Unrealized Losses | (12,402) | (856)
Short-term and Long-term Investments and Marketable Securities, Fair Value | 537,809 | 430,801
Commercial Notes Due Within Ninety Days |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Cash Equivalents, Cost Basis | 26,678 |
Cash Equivalents, Gross Unrealized Gains | 1 |
Cash Equivalents, Gross Unrealized Losses | (1) |
Cash Equivalents, Fair Value | 26,678 |
Commercial Note Due Within One Year |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Cost Basis | 142,912 | 56,627
Held to Maturity, Gross Unrealized Gains | 225 | 803
Held to Maturity, Gross Unrealized Losses | (307) |
Held to Maturity, Fair Value | 142,830 | 57,430
Commercial Note Due Within One Through Three Years |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Cost Basis | 201,590 | 195,595
Held to Maturity, Gross Unrealized Gains |  | 1,151
Held to Maturity, Gross Unrealized Losses | (5,864) | (103)
Held to Maturity, Fair Value | 195,726 | 196,643
Certificate of Deposit Due Within One Year |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Cost Basis | 50,000 |
Held to Maturity, Fair Value | 50,000 |
Certificate of Deposit Due Within One Through Two Years |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Held to Maturity, Cost Basis |  | 50,000
Held to Maturity, Fair Value |  | 50,000
Marketable Securities |  |
Schedule Of Held To Maturity Securities And Marketable Securities [Line Items] |  |
Marketable Securities, Cost Basis | 128,805 | 127,481
Marketable Securities, Gross Unrealized Losses | (6,230) | (753)
Marketable Securities, Fair Value | $ 122,575 | $ 126,728

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