Note 2 - Summary of Significant Accounting Policies (Details Textual) - USD ($)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Available-for-sale Securities	$ 229,265,000	$ 166,634,000
Available-for-sale Securities, Gross Unrealized Gain (Loss)	165,872,000	108,256,000
Deferred Tax Liabilities, Investments	64,550,000	42,250,000	$ 43,278,000
Finite-Lived Intangible Assets, Net	3,058,000	7,953,000
Finite-Lived Intangible Assets, Accumulated Amortization	$ 21,417,000	16,676,000
Finite-Lived Intangible Asset, Useful Life	5 years
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	$ 3,058,000
Finite-Lived Intangible Assets, Amortization Expense, after Year one	0
Amortization of Intangible Assets	4,895,000	5,037,000	4,907,000
Goodwill	13,400,000	13,400,000
Depreciation	$ 691,000	$ 672,000	$ 624,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period	10 years
Fair Value Inputs, Discount Rate	6.00%
Weighted Average Number of Shares Outstanding, Basic and Diluted	1,380,746	1,380,746	1,380,746
Management Incentive Plan, Future Commitment [Member]
Other Accrued Liabilities	$ 135,000	$ 62,000
Increase (Decrease) in Other Accrued Liabilities	$ 75,000	$ 15,000
Other Accrued Liabilities Impact on Earnings Per Share	$ 0.06	$ 0.01
Daily Journal Non-Consolidated [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings	5.12%	4.49%	4.13%
Management Incentive Plan Total Amount Paid	$ 268,250	$ 271,350	$ 198,915
Journal Technologies [Member]
Management Incentive Plan Total Percentage of Pre Tax Earnings	8.53%	8.30%	7.07%
Management Incentive Plan Total Amount Paid	$ 0	$ 0	$ 10,600
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%	56.00%	57.00%
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]
Deferred Set-up Costs, Current	$ 350,000	$ 350,000
Customer Relationships [Member]
Finite-Lived Intangible Assets, Net	2,776,000	7,166,000
Finite-Lived Intangible Assets, Accumulated Amortization	19,174,000	14,938,000
Developed Technology Rights [Member]
Finite-Lived Intangible Assets, Net	282,000	787,000
Finite-Lived Intangible Assets, Accumulated Amortization	$ 2,243,000	$ 1,738,000