Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	1 Months Ended	3 Months Ended				12 Months Ended
	Feb. 28, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Available-for-sale Securities, Total		$ 212,296,000				$ 212,296,000	$ 229,265,000
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax, Total		158,407,000				158,407,000	165,872,000
Deferred Tax Liabilities, Investments		42,151,000				42,151,000	64,550,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities		4,560,000				4,560,000	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax, Total						3,350,000
Finite-Lived Intangible Assets, Net, Ending Balance							3,058,000
Finite-Lived Intangible Assets, Accumulated Amortization		24,475,000				$ 24,475,000	21,417,000
Finite-Lived Intangible Asset, Useful Life						5 years
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		3,058,000				$ 3,058,000
Amortization of Intangible Assets, Total						3,058,000	4,895,000	5,037,000
Goodwill, Ending Balance		13,400,000				13,400,000	13,400,000
Depreciation, Total						$ 620,000	$ 691,000	$ 672,000
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
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets		$ 19,960,000				$ 19,960,000
Measurement Input, Discount Rate [Member]
Incentive Plan Future Commitment, Measurement Input		6.00%				6.00%
Management Incentive Plan, Future Commitment [Member]
Other Accrued Liabilities		$ 170,000				$ 170,000	$ 135,000
Increase (Decrease) in Other Accrued Liabilities						$ 35,000	$ 75,000
Other Accrued Liabilities Impact on Earnings Per Share						$ 0.03	$ 0.06
Daily Journal Non-Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings						6.09%	5.12%	4.49%
Management Incentive Plan Total Amount Paid						$ 367,400	$ 268,250	$ 271,350
Journal Technologies [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings						8.72%	8.53%	8.30%
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] | Sales Revenue, Net [Member]
Concentration Risk, Percentage						58.00%	58.00%	56.00%
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
Prepaid Expenses and Other Current Assets [Member] | New Dawn [Member]
Contract with Customer, Asset, Net, Current, Total		0				$ 0	$ 350,000
Customer Relationships [Member]
Finite-Lived Intangible Assets, Net, Ending Balance		0				0	2,776,000
Finite-Lived Intangible Assets, Accumulated Amortization		21,950,000				21,950,000	19,174,000
Developed Technology Rights [Member]
Finite-Lived Intangible Assets, Net, Ending Balance		0				0	282,000
Finite-Lived Intangible Assets, Accumulated Amortization		$ 2,525,000				$ 2,525,000	$ 2,243,000
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