Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	1 Months Ended	12 Months Ended
	Aug. 31, 2020	Sep. 30, 2020	Sep. 30, 2019
Restricted Cash and Cash Equivalents, Total		$ 2,041,000	$ 2,015,000
Net Income (Loss) Attributable to Parent, Total		4,041,000	(25,216,000)
Unrealized Gain (Loss) on Investments, Total		(3,099,000)	(17,715,000)
Equity Securities, FV-NI		179,368,000	194,581,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total		137,593,000	140,692,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Tax		35,870,000	37,241,000
Goodwill, Impairment Loss		0	13,400,000
Depreciation, Total		$ 524,000	$ 589,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year)		10 years
Weighted Average Number of Shares Outstanding, Basic and Diluted (in shares)		1,380,746	1,380,746
Operating Lease, Right-of-Use Asset		$ 140,000	$ 0
Office Lease Obligations Beyond One Year		10,000
Accrued Liabilities, Current and Noncurrent [Member]
Operating Lease, Liability, Total		$ 140,000
Measurement Input, Discount Rate [Member]
Incentive Plan Future Commitment, Measurement Input		6.00%	6.00%
Management Incentive Plan, Future Commitment [Member]
Other Accrued Liabilities		$ 1,445,000	$ 230,000
Increase (Decrease) in Other Accrued Liabilities		$ 1,215,000	$ 60,000
Other Accrued Liabilities Impact on Earnings Per Share (in dollars per share)		$ 0.88	$ 0.04
Daily Journal Non-Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings		7.51%	6.79%
Management Incentive Plan Total Amount Paid		$ 502,700	$ 465,500
Journal Technologies [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings		9.40%	9.00%
Management Incentive Plan Total Amount Paid		$ 0	$ 0
Daily Journal Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings		8.20%	8.20%
Management Incentive Plan Total Amount Paid		$ 452,900	$ 0
Product Concentration Risk [Member] | Revenue Benchmark [Member]
Concentration Risk, Percentage		71.00%	65.00%
Minimum [Member]
Property, Plant and Equipment, Useful Life (Year)		3 years
Minimum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)		5 years
Minimum [Member] | Furniture, Office Equipment, and Software [Member]
Property, Plant and Equipment, Useful Life (Year)		3 years
Minimum [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)		3 years
Maximum [Member]
Property, Plant and Equipment, Useful Life (Year)		39 years
Maximum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life (Year)		39 years
Maximum [Member] | Furniture, Office Equipment, and Software [Member]
Property, Plant and Equipment, Useful Life (Year)		5 years
Maximum [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment, Useful Life (Year)		10 years
Bonds [Member]
Proceeds from Sale of Available-for-sale Securities, Total	$ 16,307,000
Debt Securities, Available-for-sale, Realized Gain (Loss), Total	$ 4,193,000