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Marketable Securities | 9 Months Ended
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Jun. 30, 2023
Investments, Debt and Equity Securities [Abstract] |
Marketable Securities | 5. Marketable Securities The Company had sales and maturities of marketable securities of $223.3 million and $951.5 million in the three and nine months ended June 30, 2023, respectively. There were $499.8 million and $503.5 million sales and maturities of marketable securities in the three and nine months ended June 30, 2022, respectively. There were immaterial and $0.8 million realized losses on the sale and maturities of marketable securities in the three and nine months ended June 30, 2023, respectively. There were immaterial realized gains on the sale and maturities of marketable securities in the three and nine months ended June 30, 2022. Amortized cost and the fair value, including accrued interest receivable and unrealized gains (losses) on the short-term and long-term marketable securities as of June 30, 2023 and September 30, 2022 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Gross |  | Gross |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​
​ | ​ | Cost | ​ | Losses | ​ | Gains | ​ | Fair Value
June 30, 2023: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 311,618 | ​ | $ | (3,731) | ​ | $ | 1 |  | $ | 307,888
Bank certificates of deposits | ​ | ​ | 9,078 | ​ | ​ | (194) | ​ | ​ | — | ​ | ​ | 8,884
Corporate securities | ​ | ​ | 248,309 | ​ | ​ | (5,186) | ​ | ​ | — | ​ | ​ | 243,123
Municipal securities |  | ​ | 20 | ​ | ​ | (1) | ​ | ​ | — |  | ​ | 19
​ | ​ | $ | 569,025 | ​ | $ | (9,112) | ​ | $ | 1 | ​ | $ | 559,914
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
​ The fair values of the marketable securities by contractual maturities as of June 30, 2023 were as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | ​ | ​
​ | ​ | Cost | ​ | ​ | Fair Value
Due in one year or less | ​ | $ | 394,811 | ​ | $ | 390,492
Due after one year through five years | ​ |  | 171,049 | ​ |  | 166,257
Due after five years through ten years | ​ | ​ | — | ​ | ​ | —
Due after ten years | ​ |  | 3,165 | ​ |  | 3,165
Total marketable securities | ​ | $ | 569,025 | ​ | $ | 559,914
​ 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 our evaluation of the available evidence.

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