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Note 4 - Investments - Available-for-sale Investments (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Sep. 30, 2023
--- | --- | ---
Fair value | $ 134,857 | $ 136,629
Short-term US Treasury Securities [Member] |  |
Amortized cost | 123,932 | 122,534
Unrealized gains | 30 | 0
Unrealized losses | 13 | 143
Fair value | 123,949 | 122,391
Short-term Certificates of Deposit [Member] |  |
Amortized cost | 4,446 | 8,014
Unrealized gains | 0 | 0
Unrealized losses | 43 | 119
Fair value | 4,403 | 7,895
Short-term Investment [Member] |  |
Amortized cost | 128,378 | 130,548
Unrealized gains | 30 | 0
Unrealized losses | 56 | 262
Fair value | 128,352 | 130,286
US Treasury Securities [Member] |  |
Amortized cost |  | 6,719
Unrealized gains |  | 0
Unrealized losses |  | 596
Fair value |  | 6,123
Long-term Certificates of Deposit [Member] |  |
Amortized cost | 248 | 248
Unrealized gains | 0 | 0
Unrealized losses | 23 | 28
Fair value | 225 | 220
Long-term Investment [Member] |  |
Amortized cost | 6,976 | 6,967
Unrealized gains | 0 | 0
Unrealized losses | 471 | 624
Fair value | 6,505 | $ 6,343
Long-term US Treasury Securities [Member] |  |
Amortized cost | 6,728 |
Unrealized gains | 0 |
Unrealized losses | 448 |
Fair value | $ 6,280 |

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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