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Note 5 - Marketable Securities (Tables) | 12 Months Ended
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Sep. 30, 2024
Notes Tables |
Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  |  |  |
|  | Cost |  |  | Losses |  |  | Gains |  |  | Fair Value |
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 |
September 30, 2023: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 227,804 |  |  | $ | (2,573 | ) |  | $ | — |  |  | $ | 225,231 |
Bank certificates of deposits |  |  | 8,122 |  |  |  | (170 | ) |  |  | — |  |  |  | 7,952 |
Corporate securities |  |  | 221,155 |  |  |  | (4,127 | ) |  |  | — |  |  |  | 217,028 |
|  | $ | 457,081 |  |  | $ | (6,870 | ) |  | $ | — |  |  | $ | 450,211 |
Investments Classified by Contractual Maturity Date [Table Text Block] |
|  | Amortized |  |  |  |  |
|  | Cost |  |  | Fair Value |
Due in one year or less |  | $ | 151,510 |  |  | $ | 151,162 |
Due after one year through five years |  |  | 45,654 |  |  |  | 45,667 |
Due after five years through ten years |  |  | — |  |  |  | — |
Due after ten years |  |  | 3,787 |  |  |  | 3,787 |
Total marketable securities |  | $ | 200,951 |  |  | $ | 200,616 |

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