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

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).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
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Amount, before tax, of unrealized loss 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

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
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