Note 4 - Marketable Securities - Reconciliation of Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Amortized cost	$ 409,153	$ 457,081
Gross unrealized losses	(1,771)	(6,870)
Gross unrealized gains	0	0
Fair value	407,382	450,211
US Treasury and Government [Member]
Amortized cost	298,843	227,804
Gross unrealized losses	(786)	(2,573)
Gross unrealized gains	0	0
Fair value	298,057	225,231
Certificates of Deposit [Member]
Amortized cost	5,422	8,122
Gross unrealized losses	(44)	(170)
Gross unrealized gains	0	0
Fair value	5,378	7,952
Corporate Debt Securities [Member]
Amortized cost	103,869	221,155
Gross unrealized losses	(941)	(4,127)
Gross unrealized gains	0	0
Fair value	102,928	$ 217,028
US States and Political Subdivisions Debt Securities [Member]
Amortized cost	1,019
Gross unrealized losses	0
Gross unrealized gains	0
Fair value	$ 1,019