Note 4 - Net Income Per Share - Components of Common Shares Outstanding (Details) - shares	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Weighted average common shares outstanding – basic (in shares)	4,842,010	4,841,010
Dilutive effect of stock options (in shares)	8,469	5,241
Shares used in computing net income per share – diluted (in shares)	4,850,479	4,846,251