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Note 4 - Marketable Securities (Tables) | 6 Months Ended
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Mar. 31, 2025
Notes Tables |
Schedule of Available-for-Sale Securities Reconciliation [Table Text Block] |
​ |  | ​ |  |  | Gross |  |  | Gross |  |  | ​ |
​ |  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | ​ |
​ |  | Cost |  |  | Losses |  |  | Gains |  |  | Fair Value |
March 31, 2025: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities and obligations of U.S. government agencies |  | $ | 214,926 |  |  | $ | (99 | ) |  | $ | 132 |  |  | $ | 214,959 |
Bank certificates of deposit |  |  | 1,975 |  |  |  | (2 | ) |  |  | — |  |  |  | 1,973 |
Corporate securities |  |  | 25,254 |  |  |  | (31 | ) |  |  | — |  |  |  | 25,223 |
Municipal securities |  |  | 9,286 |  |  |  | — |  |  |  | 37 |  |  |  | 9,323 |
​ |  | $ | 251,441 |  |  | $ | (132 | ) |  | $ | 169 |  |  | $ | 251,478 |
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 |  | $ | 74,754 |  |  | $ | 74,697 |
Due after one year through five years |  |  | 173,099 |  |  |  | 173,193 |
Due after ten years |  |  | 3,588 |  |  |  | 3,588 |
Total marketable securities |  | $ | 251,441 |  |  | $ | 251,478 |

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