Note 6 - Shareholders' Equity (Details Textual) - USD ($)					1 Months Ended	12 Months Ended
	Jan. 25, 2019	Aug. 12, 2011	Nov. 01, 2010	Oct. 31, 2008	Apr. 30, 1998	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019	May 04, 2020	Jul. 23, 2018	Nov. 08, 2006
Preferred Stock, Shares Issued, Total (in shares)					250,000	3,000	3,000
Shares Issued, Price Per Share (in dollars per share)					$ 4.00
Payments of Stock Issuance Costs					$ 112,187
Convertible Preferred Stock, Shares Issued upon Conversion (in shares)					4.05
Preferred Stock, Liquidation Preference Per Share (in dollars per share)					$ 4.00	$ 4	$ 4
Preferred Stock, Shares Outstanding, Ending Balance (in shares)						3,000	3,000
Stock Repurchase Program, Authorized Amount											$ 20,000,000.0
Stock Repurchase Program, Additional Authorized Amount	$ 30,000,000.0	$ 20,000,000.0	$ 20,000,000.0	$ 20,000,000.0
Stock Repurchased and Retired During Period, Shares (in shares)						0	613,000	0
Stock Repurchased and Retired During Period, Value							$ 11,497,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount						$ 28,700,000
Dividends Payable, Amount Per Share (in dollars per share)						$ 0.28			$ 0.28	$ 0.27
Preferred Stock, Par or Stated Value Per Share (in dollars per share)					$ 1	$ 0.001	$ 0.001
Convertible Preferred Stock [Member]
Preferred Stock, Shares Outstanding, Ending Balance (in shares)						250,000	2,500