Note 2 - Investments (Details Textual) - USD ($)		3 Months Ended		9 Months Ended
		Jan. 31, 2021	Jan. 31, 2020	Jan. 31, 2021	Jan. 31, 2020	Apr. 30, 2020
Available-for-sale Equity Securities, Amortized Cost Basis, Total			$ 14,880,000		$ 14,880,000	$ 12,877,000
Available-for-sale Securities, Equity Securities		$ 17,516,000		$ 17,516,000		$ 14,125,000
Proceeds from Sale of Available-for-sale Securities, Equity				7,684,000
Net Income (Loss) Attributable to Parent, Total		$ 6,029,000	4,952,000	16,202,000	12,853,000
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, Portion Attributable to Parent, Total				1,245,000	592,000
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax				$ 626,000	260,000
Available-for-sale Securities, Debt Securities, Average Yield		2.34%		2.34%		2.27%
Equity Method Investments		$ 60,919,000		$ 60,919,000		$ 59,165,000
Accrued Non Voting Revenues and Non Voting Profits Interests Payable		4,671,000		4,671,000
EAM Trust [Member]
Net Income (Loss) Attributable to Parent, Total	[1]	1,260,000	$ 844,000	2,958,000	1,990,000
Fair Value of Contributed Capital at Inception						55,805,000
Cash and Liquid Securities in Excess of Working Capital Requirements Contributed to Capital Account						5,820,000
Equity Method Investment, Other than Temporary Impairment				$ 0	0
Percentage of Non Voting Profit Interest				50.00%
Accrued Non Voting Revenues and Non Voting Profits Interests Payable		4,671,000		$ 4,671,000		2,949,000
Equity Securities [Member]
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, Portion Attributable to Parent, Total				1,388,000	523,000
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax				361,000	240,000
Available-for-sale Securities, Gross Realized Gain (Loss), Total				0
AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax, Total		2,636,000		2,636,000		1,248,000
AOCI Tax, Attributable to Parent		685,000		685,000		324,000
Equity Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Net Income (Loss) Attributable to Parent, Total				1,162,000
Debt Securities [Member]
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax, Portion Attributable to Parent, Total				143,000	69,000
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax				(37,000)	20,000
AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax, Total		34,000		34,000		177,000
AOCI Tax, Attributable to Parent		$ 9,000		9,000		$ 46,000
Proceeds from Sale and Maturity of Debt Securities, Available-for-sale, Total				$ 11,431,000	$ 5,223,000

[1] Represents EAM's net income, after giving effect to Value Line's non-voting revenues interest, but before distributions to voting profits interest holders and to the Company in respect of its 50% non-voting profits interest.