Note 2 - Investments - Schedule of Carrying Value and Fair Value of Securities Available-for-sale (Details) - USD ($)	Jul. 31, 2018	Apr. 30, 2018
Cost	$ 8,385,000	$ 8,385,000
Fair value	9,917,000	9,379,000
Due within 1 year, amortized cost	8,705,000	7,868,000
Due within 1 year, gross unrealized gains	6,000	10,000
Due within 1 year, gross unrealized losses	(7,000)	(12,000)
Due within 1 year, fair value	8,704,000	7,866,000
Due 1 year through 5 years, amortized cost	250,000	600,000
Due 1 year through 5 years, gross unrealized gains
Due 1 year through 5 years, gross unrealized losses	(1,000)	(1,000)
Due 1 year through 5 years, fair value	249,000	599,000
Amortized cost	8,955,000	8,468,000
Gross unrealized gains	6,000	10,000
Gross unrealized losses	(8,000)	(13,000)
Fair value	8,953,000	8,465,000
Exchange Traded Funds [Member]
Cost	8,385,000	8,385,000
Gross unrealized gains	1,532,000	994,000
Gross Unrealized Losses
Fair value	$ 9,917,000	$ 9,379,000