Statements of Shareholders' Equity - USD ($)	Cumulative Effect, Period of Adoption, Adjustment [Member] Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Mar. 31, 2019		4,846,010
Balance at Mar. 31, 2019		$ 48,460	$ 19,910,558	$ (82,725)	$ 62,903,918	$ 82,780,211
Exercise of stock options (in shares)		2,000
Exercise of stock options		$ 20	112,340			112,360
Repurchase of common stock (in shares)		(12,972)
Repurchase of common stock		$ (130)	(687,302)			(687,432)
Unrealized gain on marketable securities, net of tax				599,248		599,248
Net income					14,526,642	14,526,642
Total comprehensive income						15,125,890
Stock-based compensation			48,360			48,360
Cash dividends declared ($4.00 per share of common stock)					(19,384,040)	(19,384,040)
Balance (in shares) at Mar. 31, 2020	4,835,038
Balance at Mar. 31, 2020		$ 48,350	19,383,956	516,523	58,046,520	77,995,349
Repurchase of common stock (in shares)		(1,806)
Repurchase of common stock		$ (18)	(91,401)			(91,419)
Unrealized gain on marketable securities, net of tax				584,596		584,596
Net income					11,694,384	11,694,384
Total comprehensive income						12,278,980
Stock-based compensation			45,572			45,572
Cash dividends declared ($4.00 per share of common stock)					(19,336,540)	(19,336,540)
Balance (in shares) at Mar. 31, 2021		4,833,232
Balance at Mar. 31, 2021		$ 48,332	$ 19,338,127	$ 1,101,119	$ 50,404,364	$ 70,891,942