Note 1 - Organization and Summary of Significant Accounting Policies - Schedule of Fair Value Measurements of Investments (Details) - USD ($)	Jan. 31, 2022	Apr. 30, 2021
Equity Securities, FV-NI, Total	$ 27,316,000	$ 23,582,000
Available-for-sale fixed income securities	597,000	2,600,000
Investments, Fair Value Disclosure, Total	56,994,000	44,391,000
Money Market Funds [Member]
Cash and cash equivalents	(29,081,000)	(18,209,000)
Cash and cash equivalents	29,081,000	18,209,000
Fair Value, Inputs, Level 1 [Member]
Equity Securities, FV-NI, Total	27,316,000	23,582,000
Available-for-sale fixed income securities	597,000	2,600,000
Investments, Fair Value Disclosure, Total	56,994,000	44,391,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and cash equivalents	(29,081,000)	(18,209,000)
Cash and cash equivalents	29,081,000	18,209,000
Fair Value, Inputs, Level 2 [Member]
Equity Securities, FV-NI, Total	0	0
Available-for-sale fixed income securities	0	0
Investments, Fair Value Disclosure, Total	0	0
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Cash and cash equivalents	0	0
Cash and cash equivalents	0	0
Fair Value, Inputs, Level 3 [Member]
Equity Securities, FV-NI, Total	0	0
Available-for-sale fixed income securities	0	0
Investments, Fair Value Disclosure, Total	0	0
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Cash and cash equivalents	0	0
Cash and cash equivalents	$ 0	$ 0