Note 6 - Changes in Shareholders' Equity and Comprehensive Income: - Schedule of Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2019	Dec. 31, 2018
Balance			$ 134,680				$ 134,680
Repurchase and retirement of common stock							(11,500)
Net income	$ 6,840			$ 7,787			18,848	$ 31,121
Balance	127,710						127,710
Additional Paid-in Capital [Member]
Balance	2,347	$ 1,617	12,478	10,881	$ 10,100	$ 9,381	12,478	9,381
Share based compensation	716	730	635	812	781	719
Dividends declared
Repurchase and retirement of common stock			(11,496)
Net income
Balance	3,063	2,347	1,617	11,693	10,881	10,100	3,063	11,693
Retained Earnings [Member]
Balance	123,215	121,997	122,172	118,930	113,751	106,320	122,172	106,320
Share based compensation
Dividends declared	(5,437)	(5,447)	(5,518)	(5,582)	(5,573)	(5,151)
Repurchase and retirement of common stock
Net income	6,840	6,665	5,343	7,787	10,752	12,582
Balance	$ 124,618	$ 123,215	$ 121,997	$ 121,135	$ 118,930	$ 113,751	$ 124,618	$ 121,135