Statements of Shareholders' Equity (Unaudited) - USD ($)	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
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
Comprehensive income: Unrealized gain on marketable securities, net of tax			(107,027)		(107,027)
Net income				3,945,049	3,945,049
Total comprehensive income					3,838,022
Cash dividends declared				(4,842,010)	(4,842,010)
Balance (in shares) at Jun. 30, 2018	4,842,010
Balance at Jun. 30, 2018	$ 48,420	19,599,298	(1,083,027)	66,873,061	85,437,752
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
Comprehensive income: Unrealized gain on marketable securities, net of tax					(55,790)
Net income					8,227,659
Total comprehensive income					8,171,869
Balance (in shares) at Sep. 30, 2018	4,844,010
Balance at Sep. 30, 2018	$ 48,440	19,817,088	(1,031,790)	66,313,661	85,147,399
Cumulative effect of accounting change			(60,365)	60,365
Balance (in shares) at Jun. 30, 2018	4,842,010
Balance at Jun. 30, 2018	$ 48,420	19,599,298	(1,083,027)	66,873,061	85,437,752
Comprehensive income: Unrealized gain on marketable securities, net of tax			51,237		51,237
Net income				4,282,610	4,282,610
Total comprehensive income					4,333,847
Cash dividends declared				(4,842,010)	(4,842,010)
Stock-based compensation		93,360			93,360
Exercise of stock options (in shares)	2,000
Exercise of stock options	$ 20	124,430			124,450
Balance (in shares) at Sep. 30, 2018	4,844,010
Balance at Sep. 30, 2018	$ 48,440	19,817,088	(1,031,790)	66,313,661	85,147,399
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: Unrealized gain on marketable securities, net of tax			570,063		570,063
Net income				3,607,419	3,607,419
Total comprehensive income					4,177,482
Cash dividends declared				(4,846,010)	(4,846,010)
Balance (in shares) at Jun. 30, 2019	4,846,010
Balance at Jun. 30, 2019	$ 48,460	19,910,558	487,338	61,665,327	82,111,683
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: Unrealized gain on marketable securities, net of tax					748,779
Net income					7,429,481
Total comprehensive income					8,178,260
Balance (in shares) at Sep. 30, 2019	4,846,010
Balance at Sep. 30, 2019	$ 48,460	19,958,918	666,054	60,641,379	81,314,811
Balance (in shares) at Jun. 30, 2019	4,846,010
Balance at Jun. 30, 2019	$ 48,460	19,910,558	487,338	61,665,327	82,111,683
Comprehensive income: Unrealized gain on marketable securities, net of tax			178,716		178,716
Net income				3,822,062	3,822,062
Total comprehensive income					4,000,778
Cash dividends declared				(4,846,010)	(4,846,010)
Stock-based compensation		48,360			48,360
Balance (in shares) at Sep. 30, 2019	4,846,010
Balance at Sep. 30, 2019	$ 48,460	$ 19,958,918	$ 666,054	$ 60,641,379	$ 81,314,811