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Note 4 - Marketable Securities | 9 Months Ended
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Jun. 30, 2025
Notes to Financial Statements |
Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] | 4. Marketable Securities The Company had sales and maturities of marketable securities of $57.9 million and $241.0 million in the three months ended June 30, 2025 and 2024, respectively. The Company had sales and maturities of marketable securities of $242.5 million and $431.5 million in the nine months ended June 30, 2025 and 2024, respectively. There were immaterial realized gains or losses in each of the three and nine months ended June 30, 2025 and 2024 on sales and maturities 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 June 30, 2025 and September 30, 2024 (in thousands):
​ |  | ​ |  |  | Gross |  |  | Gross |  |  | ​ |
​ |  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | ​ |
​ |  | Cost |  |  | Losses |  |  | Gains |  |  | Fair Value |
June 30, 2025: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 255,091 |  |  | $ | (140 | ) |  | $ | 42 |  |  | $ | 254,993 |
Bank certificates of deposit |  |  | 1,234 |  |  |  | — |  |  |  | — |  |  |  | 1,234 |
Corporate securities |  |  | 3,782 |  |  |  | — |  |  |  | — |  |  |  | 3,782 |
Municipal securities |  |  | 10,932 |  |  |  | — |  |  |  | 44 |  |  |  | 10,976 |
​ |  | $ | 271,039 |  |  | $ | (140 | ) |  | $ | 86 |  |  | $ | 270,985 |
September 30, 2024: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 118,159 |  |  | $ | (119 | ) |  | $ | 51 |  |  | $ | 118,091 |
Bank certificates of deposit |  |  | 5,212 |  |  |  | (13 | ) |  |  | 1 |  |  |  | 5,200 |
Corporate securities |  |  | 77,580 |  |  |  | (255 | ) |  |  | — |  |  |  | 77,325 |
​ |  | $ | 200,951 |  |  | $ | (387 | ) |  | $ | 52 |  |  | $ | 200,616 |
The amortized cost and fair value of the marketable securities by contractual maturities as of June 30, 2025 are presented below (in thousands):
​ |  | Amortized |  |  | ​ |
​ |  | Cost |  |  | Fair Value |
Due in one year or less |  | $ | 48,834 |  |  | $ | 48,817 |
Due after one year through five years |  |  | 218,423 |  |  |  | 218,386 |
Due after ten years |  |  | 3,782 |  |  |  | 3,782 |
Total marketable securities |  | $ | 271,039 |  |  | $ | 270,985 |
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 the evaluation of the available evidence. ​

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