Note 5 - Fair Value of Financial Instruments - Investment Securities With Unrealized Losses (Details) - USD ($)	Dec. 31, 2018	Mar. 31, 2018
Fair market value, less than 12 months	$ 17,553,361	$ 61,731,248
Gross unrealized losses, less than 12 months	(122,792)	(1,003,849)
Fair market value, 12 months or greater	46,810,042	9,072,719
Gross unrealized losses, 12 months or greater	(1,086,958)	(245,511)
Total fair market value	64,363,403	70,803,967
Total gross unrealized losses	(1,209,750)	(1,249,360)
Corporate Bond Securities [Member]
Fair market value, less than 12 months	17,553,361	61,731,248
Gross unrealized losses, less than 12 months	(122,792)	(1,003,849)
Fair market value, 12 months or greater	46,810,042	9,072,719
Gross unrealized losses, 12 months or greater	(1,086,958)	(245,511)
Total fair market value	64,363,403	70,803,967
Total gross unrealized losses	$ (1,209,750)	$ (1,249,360)