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Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Sep. 30, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 457,081 | $ 1,278,422
Gross Unrealized Losses | (6,870) | (14,660)
Gross Unrealized Gains |  | 22
Fair Value | 450,211 | 1,263,784
U.S. Treasury securities and obligations of U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 227,804 | 804,774
Gross Unrealized Losses | (2,573) | (6,163)
Gross Unrealized Gains |  | 21
Fair Value | 225,231 | 798,632
Bank certificates of deposits |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,122 | 8,335
Gross Unrealized Losses | (170) | (158)
Gross Unrealized Gains |  | 1
Fair Value | 7,952 | 8,178
Corporate securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 221,155 | 406,270
Gross Unrealized Losses | (4,127) | (8,113)
Fair Value | $ 217,028 | 398,157
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 59,043
Gross Unrealized Losses |  | (226)
Fair Value |  | $ 58,817

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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