Note 4 - Net Income Per Share - Components of Common Shares Outstanding (Details) - shares	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Weighted average common shares outstanding – basic (in shares)	4,845,010	4,841,369	4,843,355	4,841,130
Dilutive effect of stock options (in shares)	5,497	5,921	6,765	4,906
Shares used in computing net income per share – diluted (in shares)	4,850,507	4,847,290	4,850,120	4,846,036