Statements of Shareholders' Equity (Unaudited) - USD ($)	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
Balance	$ 48,420	$ 19,599,298	$ (915,635)	$ 67,709,657	$ 86,441,740
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
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	$ 48,420	19,599,298	(1,083,027)	66,873,061	85,437,752
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
Unrealized gain on marketable securities, net of tax					46,101
Net income					11,649,064
Total comprehensive income					11,695,165
Balance	$ 48,460	19,910,558	(929,899)	64,891,056	83,920,175
Balance (in shares) at Dec. 31, 2018	4,846,010
Balance at Dec. 31, 2018	$ 48,460	19,910,558	(929,899)	64,891,056	83,920,175
Balance	48,420	19,599,298	(1,083,027)	66,873,061	85,437,752
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
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
Balance	$ 48,440	19,817,088	(1,031,790)	66,313,661	85,147,399
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	48,440	19,817,088	(1,031,790)	66,313,661	85,147,399
Unrealized gain on marketable securities, net of tax			101,891		101,891
Net income				3,421,405	3,421,405
Total comprehensive income					3,523,296
Cash dividends declared				(4,844,010)	(4,844,010)
Balance	$ 48,460	19,910,558	(929,899)	64,891,056	83,920,175
Exercise of stock options (in shares)	2,000
Exercise of stock options	$ 20	93,470			93,490
Balance (in shares) at Dec. 31, 2018	4,846,010
Balance at Dec. 31, 2018	$ 48,460	19,910,558	(929,899)	64,891,056	83,920,175
Balance	48,460	19,910,558	(929,899)	64,891,056	83,920,175
Balance	$ 48,460	19,910,558	(82,725)	62,903,918	82,780,211
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
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	$ 48,460	19,910,558	487,338	61,665,327	82,111,683
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
Unrealized gain on marketable securities, net of tax					780,016
Net income					11,160,145
Total comprehensive income					11,940,161
Balance	$ 48,460	19,958,918	697,291	59,526,033	80,230,702
Balance (in shares) at Dec. 31, 2019	4,846,010
Balance at Dec. 31, 2019	$ 48,460	19,958,918	697,291	59,526,033	80,230,702
Balance	$ 48,460	19,910,558	487,338	61,665,327	82,111,683
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
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	$ 48,460	19,958,918	666,054	60,641,379	81,314,811
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	48,460	19,958,918	666,054	60,641,379	81,314,811
Unrealized gain on marketable securities, net of tax			31,237		31,237
Net income				3,730,664	3,730,664
Total comprehensive income					3,761,901
Cash dividends declared				(4,846,010)	(4,846,010)
Balance	$ 48,460	19,958,918	697,291	59,526,033	80,230,702
Balance (in shares) at Dec. 31, 2019	4,846,010
Balance at Dec. 31, 2019	$ 48,460	19,958,918	697,291	59,526,033	80,230,702
Balance	$ 48,460	$ 19,958,918	$ 697,291	$ 59,526,033	$ 80,230,702