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, 2015	4,857,953
Balance at Mar. 31, 2015	$ 48,580	$ 20,850,762	$ 746,447	$ 86,681,745	$ 108,327,534
Exercise of stock options (in shares)	18,090
Exercise of stock options	$ 181	292,728			292,909
Repurchase of common stock (in shares)	(41,033)
Repurchase of common stock	$ (411)	(2,311,262)			(2,311,673)
Comprehensive income:
Unrealized loss from marketable securities, net of tax			(295,088)		(295,088)
Net income				12,292,315	12,292,315
Total comprehensive income					11,997,227
Stock-based compensation		21,160			21,160
Tax benefit of stock-based compensation		352,294			352,294
Cash dividends declared				(19,414,864)	(19,414,864)
Exercise of stock options (in shares)	18,090
Exercise of stock options	$ 181	292,728			292,909
Balance (in shares) at Mar. 31, 2016	4,835,010
Balance at Mar. 31, 2016	$ 48,350	19,205,682	451,359	79,559,196	99,264,587
Exercise of stock options (in shares)	6,000
Exercise of stock options	$ 60	247,050			247,110
Comprehensive income:
Unrealized loss from marketable securities, net of tax			(489,657)		(489,657)
Net income				12,948,869	12,948,869
Total comprehensive income					12,459,212
Stock-based compensation		22,000			22,000
Tax benefit of stock-based compensation		32,616			32,616
Cash dividends declared				(19,346,040)	(19,346,040)
Exercise of stock options (in shares)	6,000
Exercise of stock options	$ 60	247,050			247,110
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 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