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v3.23.1

Marketable Securities (Tables) | 6 Months Ended
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Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract] |
Summary of Amortized Cost and Fair Value, Including Accrued Interest Receivable and Unrealized Holding Gains (Losses) on Short-term and Long-term Marketable Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
March 31, 2023: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 435,258 | ​ | $ | (3,672) | ​ | $ | 31 |  | $ | 431,617
Bank certificates of deposits | ​ | ​ | 10,355 | ​ | ​ | (187) | ​ | ​ | 1 | ​ | ​ | 10,169
Corporate securities | ​ | ​ | 341,868 | ​ | ​ | (5,370) | ​ | ​ | — | ​ | ​ | 336,498
Municipal securities |  | ​ | 1,544 | ​ | ​ | (1) | ​ | ​ | — |  | ​ | 1,543
​ | ​ | $ | 789,025 | ​ | $ | (9,230) | ​ | $ | 32 | ​ | $ | 779,827
September 30, 2022: | ​ | ​ |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. Treasury securities and obligations of U.S. government agencies | ​ | $ | 804,774 |  | $ | (6,163) |  | $ | 21 | ​ | $ | 798,632
Bank certificates of deposits | ​ | ​ | 8,335 | ​ | ​ | (158) | ​ | ​ | 1 | ​ | ​ | 8,178
Corporate securities | ​ | ​ | 406,270 | ​ | ​ | (8,113) | ​ | ​ | — | ​ | ​ | 398,157
Municipal securities | ​ |  | 59,043 | ​ | ​ | (226) | ​ | ​ | — |  | ​ | 58,817
​ | ​ | $ | 1,278,422 | ​ | $ | (14,660) | ​ | $ | 22 | ​ | $ | 1,263,784
​
Fair Value of Marketable Securities by Contractual Maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Amortized | ​ | ​ | ​
​ |  | ​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | ​ | ​ | $ | 515,364 | ​ | $ | 513,652
Due after one year through five years | ​ | ​ | ​ |  | 270,591 | ​ |  | 263,105
Due after five years through ten years | ​ | ​ | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ | ​ | ​ |  | 3,070 | ​ |  | 3,070
Total marketable securities | ​ | ​ | ​ | $ | 789,025 | ​ | $ | 779,827
​

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