Statements of Shareholders' Equity - USD ($)	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
Balance (in shares) at Mar. 31, 2017	4,841,010
Balance at Mar. 31, 2017	$ 48,410	$ 19,507,348	$ (38,298)	$ 73,162,025	$ 92,679,485
Exercise of stock options (in shares)	1,000
Exercise of stock options	$ 10	51,030			51,040
Comprehensive income:
Unrealized gain (loss) from marketable securities, net of tax			(877,337)		(877,337)
Net income				13,912,672	13,912,672
Total comprehensive income					13,035,335
Stock-based compensation		40,920			40,920
Cash dividends declared				(19,365,040)	(19,365,040)
Exercise of stock options (in shares)	1,000
Exercise of stock options	$ 10	51,030			51,040
Balance (in shares) at Mar. 31, 2018	4,842,010
Balance at Mar. 31, 2018	$ 48,420	19,599,298	(915,635)	67,709,657	86,441,740
Exercise of stock options (in shares)	4,000
Exercise of stock options	$ 40	217,900			217,940
Comprehensive income:
Unrealized gain (loss) from marketable securities, net of tax			893,275		893,275
Net income				14,507,936	14,507,936
Total comprehensive income					15,401,211
Stock-based compensation		93,360			93,360
Cash dividends declared				(19,374,040)	(19,374,040)
Exercise of stock options (in shares)	4,000
Exercise of stock options	$ 40	217,900			217,940
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
Comprehensive income:
Cumulative effect of accounting change			$ (60,365)	$ 60,365