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Note 5 - Marketable Securities (Tables) | 12 Months Ended
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Sep. 30, 2025
Notes Tables |
Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  |  |  |
|  | Cost |  |  | Losses |  |  | Gains |  |  | Fair Value |
September 30, 2025: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 245,691 |  |  | $ | (94 | ) |  | $ | 168 |  |  | $ | 245,765 |
Bank certificates of deposits |  |  | 1,640 |  |  |  | — |  |  |  | 1 |  |  |  | 1,641 |
Corporate securities |  |  | 4,199 |  |  |  | — |  |  |  | — |  |  |  | 4,199 |
Municipal securities |  |  | 11,048 |  |  |  | — |  |  |  | 69 |  |  |  | 11,117 |
|  | $ | 262,578 |  |  | $ | (94 | ) |  | $ | 238 |  |  | $ | 262,722 |
September 30, 2024: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 118,159 |  |  | $ | (119 | ) |  | $ | 51 |  |  | $ | 118,091 |
Bank certificates of deposits |  |  | 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 |  | $ | 61,116 |  |  | $ | 61,137 |
Due after one year through five years |  |  | 197,511 |  |  |  | 197,634 |
Due after five years through ten years |  |  | — |  |  |  | — |
Due after ten years |  |  | 3,951 |  |  |  | 3,951 |
Total marketable securities |  | $ | 262,578 |  |  | $ | 262,722 |

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