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Note 4 - Marketable Securities (Tables) | 9 Months Ended
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Jun. 30, 2025
Notes Tables |
Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] |
​ |  | ​ |  |  | 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 |
Investments Classified by Contractual Maturity Date [Table Text Block] |
​ |  | 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 |

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