Statements of Shareholders' Equity - USD ($)	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
Comprehensive income:
Balance (in shares)	4,851,043
Balance (in shares) at Mar. 31, 2014	4,851,043
Balance at Mar. 31, 2014	$ 48,510	$ 20,464,883	$ 877,857	$ 82,313,391	$ 103,704,641
Exercise of stock options (in shares)	6,910
Exercise of stock options	$ 70	302,631			302,701
Comprehensive income:
Unrealized loss from marketable securities, net of tax			(131,410)		(131,410)
Net income				14,368,354	14,368,354
Total comprehensive income					14,236,944
Stock-based compensation		58,960			58,960
Tax benefit of stock-based compensation		24,288			24,288
Cash dividends declared				(10,000,000)	(10,000,000)
Balance (in shares)	4,851,043
Balance at Mar. 31, 2015	$ 48,580	20,850,762	746,447	86,681,745	108,327,534
Comprehensive income:
Exercise of stock options (in shares)	6,910
Exercise of stock options	$ 70	302,631			302,701
Repurchase of common stock					0
Balance (in shares) at Mar. 31, 2015	4,857,953
Comprehensive income:
Balance (in shares)	4,857,953
Exercise of stock options (in shares)	18,090
Exercise of stock options	$ 181	292,728			292,909
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)
Balance (in shares)	4,857,953
Balance at Mar. 31, 2016	$ 48,350	19,205,682	451,359	79,559,196	99,264,587
Comprehensive income:
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)			(1,695,788)
Balance (in shares) at Mar. 31, 2016	4,835,010
Comprehensive income:
Balance (in shares)	4,835,010
Exercise of stock options (in shares)	6,000
Exercise of stock options	$ 60	247,050			247,110
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)
Balance (in shares)	4,835,010
Balance at Mar. 31, 2017	$ 48,410	19,507,348	$ (38,298)	$ 73,162,025	92,679,485
Comprehensive income:
Exercise of stock options (in shares)	6,000
Exercise of stock options	$ 60	$ 247,050			247,110
Repurchase of common stock					$ 0
Balance (in shares) at Mar. 31, 2017	4,841,010
Comprehensive income:
Balance (in shares)	4,841,010