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Marketable Securities | 3 Months Ended
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Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | 4. Marketable Securities The Company had sales and maturities of marketable securities of $110.3 million and $607.2 million in the three months ended December 31, 2023 and 2022, respectively. There were immaterial and $0.3 million realized losses on the sale and maturities of marketable securities in the three months ended December 31, 2023 and 2022, respectively. 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 December 31, 2023 and September 30, 2023 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
December 31, 2023: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 148,119 | ​ | $ | (1,200) | ​ | $ | — |  | $ | 146,919
Bank certificates of deposit | ​ | ​ | 7,842 | ​ | ​ | (94) | ​ | ​ | — | ​ | ​ | 7,748
Corporate securities | ​ | ​ | 190,696 | ​ | ​ | (2,189) | ​ | ​ | — | ​ | ​ | 188,507
​ | ​ | $ | 346,657 | ​ | $ | (3,483) | ​ | $ | — | ​ | $ | 343,174
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 December 31, 2023 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | ​ | ​
​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | $ | 283,849 | ​ | $ | 281,212
Due after one year through five years | ​ |  | 59,486 | ​ |  | 58,640
Due after five years through ten years | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ |  | 3,322 | ​ |  | 3,322
Total marketable securities | ​ | $ | 346,657 | ​ | $ | 343,174
​ 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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