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Marketable Securities (Tables) | 9 Months Ended
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Jun. 30, 2022
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 | The following is a summary of the amortized cost and the fair value, including accrued interest receivable as well as unrealized holding gains (losses) on the short-term and long-term marketable securities as of June 30, 2022 and September 30, 2021 (in thousands):
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
June 30, 2022: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 536,322 | ​ | $ | (2,981) | ​ | $ | 93 |  | $ | 533,434
Bank certificates of deposits | ​ | ​ | 6,341 | ​ | ​ | (45) | ​ | ​ | — | ​ | ​ | 6,296
Corporate securities | ​ | ​ | 372,922 | ​ | ​ | (4,009) | ​ | ​ | 159 | ​ | ​ | 369,072
Municipal securities |  | ​ | 112,621 | ​ | ​ | (334) | ​ | ​ | 1 |  | ​ | 112,288
​ | ​ | $ | 1,028,206 | ​ | $ | (7,369) | ​ | $ | 253 | ​ | $ | 1,021,090
September 30, 2021: | ​ | ​ |  | ​ |  |  | ​ |  |  | ​ |  |
Bank certificates of deposits | ​ | $ | 30 | ​ | $ | — | ​ | $ | — | ​ | $ | 30
Corporate securities | ​ | ​ | 3,624 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 3,624
Municipal securities | ​ |  | — | ​ | ​ | — | ​ | ​ | — |  | ​ | —
Other debt securities | ​ |  | 25 | ​ | ​ | — | ​ | ​ | — |  | ​ | 25
​ | ​ | $ | 3,679 | ​ | $ | — | ​ | $ | — | ​ | $ | 3,679
​
Fair Value of Marketable Securities by Contractual Maturity | The fair values of the marketable securities by contractual maturities at June 30, 2022 are presented below (in thousands):
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Amortized | ​ | ​ | ​
​ |  | ​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | ​ | ​ | $ | 711,599 | ​ | $ | 708,895
Due after one year through five years | ​ | ​ | ​ |  | 313,633 | ​ |  | 309,221
Due after five years through ten years | ​ | ​ | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ | ​ | ​ |  | 2,974 | ​ |  | 2,974
Total marketable securities | ​ | ​ | ​ | $ | 1,028,206 | ​ | $ | 1,021,090

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