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Marketable Securities - Summary of Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Sep. 30, 2021
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Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 1,028,206 | $ 3,679
Gross Unrealized Losses | (7,369) |
Gross Unrealized Gains | 253 |
Fair Value | 1,021,090 | 3,679
U.S. Treasury securities and obligations of U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 536,322 |
Gross Unrealized Losses | (2,981) |
Gross Unrealized Gains | 93 |
Fair Value | 533,434 |
Bank certificates of deposits |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 6,341 | 30
Gross Unrealized Losses | (45) |
Fair Value | 6,296 | 30
Corporate securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 372,922 | 3,624
Gross Unrealized Losses | (4,009) |
Gross Unrealized Gains | 159 |
Fair Value | 369,072 | 3,624
Municipal securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 112,621 |
Gross Unrealized Losses | (334) |
Gross Unrealized Gains | 1 |
Fair Value | $ 112,288 |
Other debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 25
Fair Value |  | $ 25

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563 + Details
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