Note 6 - Fair Value Measurements - Fair Value by Major Security Type (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Short-term marketable securities	$ 5,976	$ 4,265
Long-term Securities	2,297	3,805
Fair Value, Nonrecurring [Member]
Cost Basis	8,479	8,428
Unrealized Gains (Losses)	2	7
Fair Value	8,481	8,435
Cash Equivalents	208	365
Short-term marketable securities	5,976	4,265
Long-term Securities	2,297	3,805
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member]
Cost Basis	208	365
Unrealized Gains (Losses)	0	0
Fair Value	208	365
Cash Equivalents	208	365
Short-term marketable securities	0	0
Long-term Securities	0	0
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Cost Basis	8,271	8,063
Unrealized Gains (Losses)	2	7
Fair Value	8,273	8,070
Cash Equivalents	0	0
Short-term marketable securities	5,976	4,265
Long-term Securities	2,297	3,805
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member] | Certificates of Deposit [Member]
Cost Basis	1,193	1,195
Unrealized Gains (Losses)	0	0
Fair Value	1,193	1,195
Cash Equivalents	0	0
Short-term marketable securities	469	0
Long-term Securities	724	1,195
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Cost Basis	5,879	3,777
Unrealized Gains (Losses)	2	4
Fair Value	5,881	3,781
Cash Equivalents	0	0
Short-term marketable securities	4,451	2,431
Long-term Securities	1,430	1,350
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member] | Corporate Debt Securities [Member]
Cost Basis	1,199	3,091
Unrealized Gains (Losses)	0	3
Fair Value	1,199	3,094
Cash Equivalents	0	0
Short-term marketable securities	1,056	1,834
Long-term Securities	$ 143	$ 1,260