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) |