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Note 4 - Investments - Securities in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2023 | Sep. 30, 2023
--- | --- | ---
Fair value unrealized loss, less than 12 months | $ 38,452 | $ 113,153
Gross unrealized loss, less than 12 months | 50 | 131
Fair value unrealized loss, greater than 12 months | 10,664 | 23,476
Gross unrealized loss, greater than 12 months | 509 | 755
US Treasury Securities [Member] |  |
Fair value unrealized loss, less than 12 months | 38,207 | 112,908
Gross unrealized loss, less than 12 months | 50 | 131
Fair value unrealized loss, greater than 12 months | 6,280 | 15,606
Gross unrealized loss, greater than 12 months | 444 | 608
Certificates of Deposit [Member] |  |
Fair value unrealized loss, less than 12 months | 245 | 245
Gross unrealized loss, less than 12 months | 0 | 0
Fair value unrealized loss, greater than 12 months | 4,384 | 7,870
Gross unrealized loss, greater than 12 months | $ 65 | $ 147

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