Note 6 - Fair Value Measurements - Unrealized Loss on Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Sep. 30, 2023
Unrealized loss position, less than 12 months, fair value	$ 1,236	$ 684
Unrealized loss position, less than 12 months, gross unrealized loss	(2)	(2)
Unrealized loss position, more than 12 months, fair value	362	496
Unrealized loss position, more than 12 months, gross unrealized loss	(1)	(8)
Unrealized loss position, fair value	1,598	10
Unrealized loss position, gross unrealized loss	(3)	(1,180)
Short-term Securities	3	1,180
Long-term Securities	(1,598)	(10)
Certificates of Deposit [Member]
Unrealized loss position, less than 12 months, fair value	0	0
Unrealized loss position, less than 12 months, gross unrealized loss	0	0
Unrealized loss position, more than 12 months, fair value	0	0
Unrealized loss position, more than 12 months, gross unrealized loss	0	0
Unrealized loss position, fair value	0	0
Unrealized loss position, gross unrealized loss	0	0
Short-term Securities	(0)	(0)
Long-term Securities	0	0
US Treasury and Government [Member]
Unrealized loss position, less than 12 months, fair value	0	0
Unrealized loss position, less than 12 months, gross unrealized loss	0	0
Unrealized loss position, more than 12 months, fair value	0	0
Unrealized loss position, more than 12 months, gross unrealized loss	0	0
Unrealized loss position, fair value	0	0
Unrealized loss position, gross unrealized loss	0	0
Short-term Securities	(0)	(0)
Long-term Securities	0	0
US States and Political Subdivisions Debt Securities [Member]
Unrealized loss position, less than 12 months, fair value	243	684
Unrealized loss position, less than 12 months, gross unrealized loss	(1)	(2)
Unrealized loss position, more than 12 months, fair value	0	235
Unrealized loss position, more than 12 months, gross unrealized loss	0	(5)
Unrealized loss position, fair value	243	919
Unrealized loss position, gross unrealized loss	(1)	(7)
Short-term Securities	1	7
Long-term Securities	(243)	(919)
Corporate Debt Securities [Member]
Unrealized loss position, less than 12 months, fair value	993	0
Unrealized loss position, less than 12 months, gross unrealized loss	(1)	0
Unrealized loss position, more than 12 months, fair value	362	261
Unrealized loss position, more than 12 months, gross unrealized loss	(1)	(3)
Unrealized loss position, fair value	1,355	261
Unrealized loss position, gross unrealized loss	(2)	(3)
Short-term Securities	2	3
Long-term Securities	$ (1,355)	$ (261)