Condensed Consolidated Statements of Shareholders' Equity - USD ($)	Common stock	Treasury stock	Additional paid-in capital	Retained earnings	Total
Balance at Mar. 31, 2019	$ 588,826	$ (36,484,561)	$ 863,583,692	$ 84,994,601	$ 912,682,558
Balance (in shares) at Mar. 31, 2019	58,882,515
Increase (Decrease) in Shareholders' Equity
Net income for the period				6,075,059	6,075,059
Restricted share award issuances	$ 78		(78)
Restricted share award issuances (in shares)	7,750
Stock-based compensation			1,305,827		1,305,827
Purchase of treasury stock		(983,582)			(983,582)
Balance at Jun. 30, 2019	$ 588,904	(37,468,143)	864,889,441	91,069,660	919,079,862
Balance (in shares) at Jun. 30, 2019	58,890,265
Balance at Mar. 31, 2019	$ 588,826	(36,484,561)	863,583,692	84,994,601	912,682,558
Balance (in shares) at Mar. 31, 2019	58,882,515
Increase (Decrease) in Shareholders' Equity
Net income for the period					82,415,867
Balance at Dec. 31, 2019	$ 590,783	(52,406,243)	866,429,768	167,410,468	982,024,776
Balance (in shares) at Dec. 31, 2019	59,078,230
Balance at Jun. 30, 2019	$ 588,904	(37,468,143)	864,889,441	91,069,660	919,079,862
Balance (in shares) at Jun. 30, 2019	58,890,265
Increase (Decrease) in Shareholders' Equity
Net income for the period				40,711,896	40,711,896
Restricted share award issuances	$ 1,832		(1,832)
Restricted share award issuances (in shares)	183,220
Stock-based compensation			890,700		890,700
Purchase of treasury stock		(6,310,514)			(6,310,514)
Balance at Sep. 30, 2019	$ 590,736	(43,778,657)	865,778,309	131,781,556	954,371,944
Balance (in shares) at Sep. 30, 2019	59,073,485
Increase (Decrease) in Shareholders' Equity
Net income for the period				35,628,912	35,628,912
Restricted share award issuances	$ 47		(47)
Restricted share award issuances (in shares)	4,745
Stock-based compensation			651,506		651,506
Purchase of treasury stock		(8,627,586)			(8,627,586)
Balance at Dec. 31, 2019	$ 590,783	(52,406,243)	866,429,768	167,410,468	982,024,776
Balance (in shares) at Dec. 31, 2019	59,078,230
Balance at Mar. 31, 2020	$ 590,833	(87,183,865)	866,809,371	196,835,859	977,052,198
Balance (in shares) at Mar. 31, 2020	59,083,290
Increase (Decrease) in Shareholders' Equity
Net income for the period				12,168,005	12,168,005
Restricted share award issuances	$ 3,516		(3,516)
Restricted share award issuances (in shares)	351,629
Stock-based compensation			1,930,902		1,930,902
Purchase of treasury stock		(1,198,214)			(1,198,214)
Balance at Jun. 30, 2020	$ 594,349	(88,382,079)	868,736,757	209,003,864	989,952,891
Balance (in shares) at Jun. 30, 2020	59,434,919
Balance at Mar. 31, 2020	$ 590,833	(87,183,865)	866,809,371	196,835,859	977,052,198
Balance (in shares) at Mar. 31, 2020	59,083,290
Increase (Decrease) in Shareholders' Equity
Net income for the period					48,531,219
Balance at Dec. 31, 2020	$ 594,651	(99,862,114)	869,673,244	245,367,078	1,015,772,859
Balance (in shares) at Dec. 31, 2020	59,465,124
Balance at Jun. 30, 2020	$ 594,349	(88,382,079)	868,736,757	209,003,864	989,952,891
Balance (in shares) at Jun. 30, 2020	59,434,919
Increase (Decrease) in Shareholders' Equity
Net income for the period				537,950	537,950
Restricted share award issuances	$ 151		(151)
Restricted share award issuances (in shares)	15,100
Stock-based compensation			406,721		406,721
Purchase of treasury stock		(1,306,388)			(1,306,388)
Balance at Sep. 30, 2020	$ 594,500	(89,688,467)	869,143,327	209,541,814	989,591,174
Balance (in shares) at Sep. 30, 2020	59,450,019
Increase (Decrease) in Shareholders' Equity
Net income for the period				35,825,264	35,825,264
Restricted share award issuances	$ 151		(151)
Restricted share award issuances (in shares)	15,105
Stock-based compensation			530,068		530,068
Purchase of treasury stock		(10,173,647)			(10,173,647)
Balance at Dec. 31, 2020	$ 594,651	$ (99,862,114)	$ 869,673,244	$ 245,367,078	$ 1,015,772,859
Balance (in shares) at Dec. 31, 2020	59,465,124