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Marketable Securities | 6 Months Ended
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Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | 4. Marketable Securities The Company had sales and maturities of marketable securities of $80.2 million and $121.0 million in the three months ended March 31, 2024 and 2023, respectively. The Company had sales and maturities of marketable securities of $190.5 million and $728.2 million in the six months ended March 31, 2024 and 2023, respectively. There were insignificant realized gains or losses in each of the three and six months ended March 31, 2024 and 2023 on the sale and maturity of marketable securities. The following is a summary of the amortized cost and the fair value, including accrued interest receivable as well as unrealized gains (losses) on the short-term and long-term marketable securities as of March 31, 2024 and September 30, 2023 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
March 31, 2024: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 433,188 | ​ | $ | (902) | ​ | $ | 8 |  | $ | 432,294
Bank certificates of deposit | ​ | ​ | 7,870 | ​ | ​ | (68) | ​ | ​ | — | ​ | ​ | 7,802
Corporate securities | ​ | ​ | 171,648 | ​ | ​ | (1,515) | ​ | ​ | 3 | ​ | ​ | 170,136
Municipal securities |  | ​ | 1,006 | ​ | ​ | — | ​ | ​ | — |  | ​ | 1,006
​ | ​ | $ | 613,712 | ​ | $ | (2,485) | ​ | $ | 11 | ​ | $ | 611,238
September 30, 2023: | ​ | ​ |  | ​ |  |  | ​ |  |  | ​ |  |
U.S. Treasury securities and obligations of U.S. government agencies | ​ | $ | 227,804 |  | $ | (2,573) |  | $ | — | ​ | $ | 225,231
Bank certificates of deposit | ​ | ​ | 8,122 | ​ | ​ | (170) | ​ | ​ | — | ​ | ​ | 7,952
Corporate securities | ​ | ​ | 221,155 | ​ | ​ | (4,127) | ​ | ​ | — | ​ | ​ | 217,028
​ | ​ | $ | 457,081 | ​ | $ | (6,870) | ​ | $ | — | ​ | $ | 450,211
​ The fair values of the marketable securities by contractual maturities as of March 31, 2024 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | ​ | ​
​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | $ | 470,012 | ​ | $ | 468,220
Due after one year through five years | ​ |  | 140,222 | ​ |  | 139,540
Due after five years through ten years | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ |  | 3,478 | ​ |  | 3,478
Total marketable securities | ​ | $ | 613,712 | ​ | $ | 611,238
​ Expected maturities could differ from contractual maturities because the security issuers may have the right to prepay obligations without prepayment penalties. Unrealized losses from fixed-income securities are primarily attributable to changes in interest rates. The Company does not believe any unrealized losses represent impairments based on its evaluation of the available evidence.

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