Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	1 Months Ended		12 Months Ended
	Jan. 31, 2021	Aug. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Restricted Cash and Cash Equivalents, Total			$ 2,043,000	$ 2,041,000
Net Income (Loss) Attributable to Parent, Total			112,900,000	4,041,000
Unrealized Gain (Loss) on Investments, Total			106,499,000	(3,099,000)
Equity Securities, FV-NI, Current			347,573,000	179,368,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total			244,093,000	137,593,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Tax			64,115,000	35,870,000
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale, Total	$ 45,033,000		45,033,000	16,307,000
Debt Securities, Available-for-sale, Realized Gain (Loss), Total	41,749,000
Payments to Acquire Marketable Securities, Total	$ 64,990,000		64,990,000
Depreciation, Total			$ 480,000	$ 524,000
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			$ 215,000	$ 140,000
Accrued Liabilities, Current and Noncurrent [Member]
Operating Lease, Liability, Total			$ 103,000
Measurement Input, Discount Rate [Member]
Incentive Plan Future Commitment, Measurement Input			6.00%
Management Incentive Plan, Future Commitment [Member]
Other Accrued Liabilities			$ 3,280,000	1,445,000
Increase (Decrease) in Other Accrued Liabilities			$ 1,835,000	$ 1,215,000
Other Accrued Liabilities Impact on Earnings Per Share (in dollars per share)			$ 1.33	$ 0.88
Daily Journal Non-Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings			4.96%	7.51%
Management Incentive Plan Total Amount Paid			$ 332,940	$ 502,700
Journal Technologies [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings			12.33%	9.78%
Management Incentive Plan Total Amount Paid			$ 255,300	$ 0
Daily Journal Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings			12.24%	8.20%
Management Incentive Plan Total Amount Paid			$ 1,049,750	$ 452,900
Product Concentration Risk [Member] | Revenue Benchmark [Member] | License and Service [Member]
Concentration Risk, Percentage			70.00%	71.00%
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 and Maturity of Debt Securities, Available-for-sale, Total		$ 16,307,000
Debt Securities, Available-for-sale, Realized Gain (Loss), Total		$ 4,193,000