Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	1 Months Ended	3 Months Ended								12 Months Ended
	Feb. 28, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2018
Restricted Cash and Cash Equivalents, Total		$ 2,015,000								$ 2,015,000
Net Income (Loss) Attributable to Parent, Total		(21,533,000)	$ 5,018,000	$ 3,823,000	$ (12,524,000)	$ 14,739,000	$ (232,000)	$ (972,000)	$ (5,334,000)	(25,216,000)	$ 8,201,000	$ (918,000)
Unrealized Gain (Loss) on Investments, Total		(28,640,000)	$ 8,497,000	$ 3,214,000	$ (786,000)					(17,715,000)
Equity Securities, FV-NI		194,581,000								194,581,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Total										140,692,000
Equity Securities, FV-NI, Unrealized Gain (Loss), Tax										37,241,000
Available-for-sale Securities, Total						212,296,000					212,296,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax, Total						158,407,000					158,407,000
Deferred Tax Liabilities, Investments		37,241,000				42,151,000				37,241,000	42,151,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities									$ 4,560,000		4,560,000
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax, Total											3,350,000
Finite-Lived Intangible Asset, Useful Life										5 years
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		0				3,058,000				$ 0	3,058,000	4,895,000
Goodwill, Impairment Loss		13,400,000								13,400,000
Depreciation, Total										$ 589,000	$ 620,000	$ 691,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period										10 years
Weighted Average Number of Shares Outstanding, Basic and Diluted										1,380,746	1,380,746	1,380,746
Cumulative Effect of New Accounting Principle in Period of Adoption
Operating Lease, Liability, Current		$ 40,000								$ 40,000
AOCI Attributable to Parent [Member]
Net Income (Loss) Attributable to Parent, Total
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax, Total											3,350,000
Cumulative Effect of New Accounting Principle in Period of Adoption						$ (115,786,000)					$ (115,786,000)
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption													$ 115,786,000
Measurement Input, Discount Rate [Member]
Incentive Plan Future Commitment, Measurement Input		6.00%				6.00%				6.00%	6.00%
Management Incentive Plan, Future Commitment [Member]
Other Accrued Liabilities		$ 230,000				$ 170,000				$ 230,000	$ 170,000
Increase (Decrease) in Other Accrued Liabilities										$ 60,000	$ 35,000
Other Accrued Liabilities Impact on Earnings Per Share										$ 0.04	$ 0.03
Daily Journal Non-Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings										6.79%	6.09%	5.12%
Management Incentive Plan Total Amount Paid										$ 465,500	$ 367,400	$ 268,250
Journal Technologies [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings										9.00%	8.72%	8.53%
Management Incentive Plan Total Amount Paid										$ 0	$ 0	$ 0
Daily Journal Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings										8.20%	8.20%	8.20%
Management Incentive Plan Total Amount Paid										$ 0	$ 0	$ 0
Product Concentration Risk [Member] | Revenue Benchmark [Member]
Concentration Risk, Percentage										65.00%	58.00%	58.00%
Minimum [Member]
Property, Plant and Equipment, Useful Life										3 years
Minimum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life										5 years
Minimum [Member] | Furniture, Office Equipment, and Software [Member]
Property, Plant and Equipment, Useful Life										3 years
Minimum [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment, Useful Life										3 years
Maximum [Member]
Property, Plant and Equipment, Useful Life										39 years
Maximum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment, Useful Life										39 years
Maximum [Member] | Furniture, Office Equipment, and Software [Member]
Property, Plant and Equipment, Useful Life										5 years
Maximum [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment, Useful Life										10 years
Customer Relationships [Member]
Finite-Lived Intangible Assets, Net, Ending Balance		0				0				$ 0	$ 0
Finite-Lived Intangible Assets, Accumulated Amortization		0				21,950,000				0	21,950,000
Developed Technology Rights [Member]
Finite-Lived Intangible Assets, Net, Ending Balance		0				0				0	0
Finite-Lived Intangible Assets, Accumulated Amortization		$ 0				$ 2,525,000				$ 0	$ 2,525,000
Bonds [Member]
Proceeds from Sale of Available-for-sale Securities, Total	$ 8,125,000
Debt Securities, Available-for-sale, Realized Gain (Loss), Total	3,180,000
Reclassification of Tax Effect from AOCI to Retained Earnings	$ 30,000