Statements of Shareholders' Equity (Unaudited) - USD ($)	Cumulative Effect, Period of Adoption, Adjustment [Member] Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Total
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 (loss) gain from 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 ($1.00 per share of common stock)					(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
Unrealized (loss) gain from marketable securities, net of tax						780,016
Net income						11,160,145
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 (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 (loss) gain from marketable securities, net of tax				178,716		178,716
Net income					3,822,062	3,822,062
Total comprehensive income						4,000,778
Stock-based compensation			48,360			48,360
Cash dividends declared ($1.00 per share of common stock)					(4,846,010)	(4,846,010)
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
Unrealized (loss) gain from 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 ($1.00 per share of common stock)					(4,846,010)	(4,846,010)
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 (in shares) at Mar. 31, 2020		4,835,038
Balance at Mar. 31, 2020		$ 48,350	19,383,956	516,523	58,046,520	77,995,349	[1]
Unrealized (loss) gain from marketable securities, net of tax				1,242,662		1,242,662
Net income					2,411,865	2,411,865
Total comprehensive income						3,654,527
Stock-based compensation			2,707			2,707
Cash dividends declared ($1.00 per share of common stock)					(4,835,038)	(4,835,038)
Balance (in shares) at Jun. 30, 2020	4,835,038
Balance at Jun. 30, 2020		$ 48,350	19,386,663	1,759,185	55,623,347	76,817,545
Balance (in shares) at Mar. 31, 2020		4,835,038
Balance at Mar. 31, 2020		$ 48,350	19,383,956	516,523	58,046,520	77,995,349	[1]
Unrealized (loss) gain from marketable securities, net of tax						966,886
Net income						8,559,914
Balance (in shares) at Dec. 31, 2020		4,833,232
Balance at Dec. 31, 2020		$ 48,332	19,333,852	1,483,409	52,103,126	72,968,719
Balance (in shares) at Jun. 30, 2020	4,835,038
Balance at Jun. 30, 2020		48,350	19,386,663	1,759,185	55,623,347	76,817,545
Unrealized (loss) gain from marketable securities, net of tax				(130,324)		(130,324)
Net income					2,222,340	2,222,340
Total comprehensive income						2,092,016
Stock-based compensation			34,315			34,315
Cash dividends declared ($1.00 per share of common stock)					(4,835,038)	(4,835,038)
Repurchase of common stock (in shares)		(1,806)
Repurchase of common stock		$ (18)	(91,401)			(91,419)
Balance (in shares) at Sep. 30, 2020		4,833,232
Balance at Sep. 30, 2020		$ 48,332	19,329,577	1,628,861	53,010,649	74,017,419
Unrealized (loss) gain from marketable securities, net of tax				(145,452)		(145,452)
Net income					3,925,709	3,925,709
Total comprehensive income						3,780,257
Stock-based compensation			4,275			4,275
Cash dividends declared ($1.00 per share of common stock)					(4,833,232)	(4,833,232)
Balance (in shares) at Dec. 31, 2020		4,833,232
Balance at Dec. 31, 2020		$ 48,332	$ 19,333,852	$ 1,483,409	$ 52,103,126	$ 72,968,719

[1] The March 31, 2020 Balance Sheet is derived from the audited financial statements contained in our Annual Report on Form 10-K for the fiscal year ended March 31, 2020.