Note 2 - Investments - Schedule of Carrying Value and Fair Value of Securities Available-for-sale (Details) - USD ($)	Oct. 31, 2017	Apr. 30, 2017
Cost	$ 8,385,000	$ 8,385,000
Fair value	9,494,000	9,097,000
Due within 1 year, amortized cost	7,126,000	4,384,000
Due within 1 year, gross unrealized gains	3,000	4,000
Due within 1 year, gross unrealized losses	(8,000)	(3,000)
Due within 1 year, fair value	7,121,000	4,385,000
Due 1 year through 5 years, amortized cost	1,350,000	3,100,000
Due 1 year through 5 years, gross unrealized gains
Due 1 year through 5 years, gross unrealized losses	(1,000)	(6,000)
Due 1 year through 5 years, fair value	1,349,000	3,094,000
Amortized cost	8,476,000	7,484,000
Gross unrealized gains	3,000	4,000
Gross unrealized losses	(9,000)	(9,000)
Fair value	8,470,000	7,479,000
Exchange Traded Funds [Member]
Cost	8,385,000	8,385,000
Gross unrealized gains	1,109,000	712,000
Fair value	$ 9,494,000	$ 9,097,000