Unaudited Condensed Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended
	Mar. 31, 2021		Mar. 31, 2020
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 1,963,943,128	[1]	$ 1,938,087,820
Issuance of common stock from stock plan transactions	$ 9,265,840		7,355,488
Repurchases of common stock (in shares)	(2,800,000)
Repurchases of common stock	$ (97,692,763)		(178,819,584)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	5,894,645		6,356,070
Dividends declared	(28,984,621)		(29,383,859)
Net income	113,450,870		89,505,801
Other comprehensive (loss) income	(4,896,385)		1,503,897
Balance at end of period	$ 1,960,980,714		$ 1,834,605,633
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	243,692,869		251,277,515
Balance at beginning of period	$ 14,621,572		$ 15,076,651
Issuance of common stock from stock plan transactions (in shares)	601,020		607,175
Issuance of common stock from stock plan transactions	$ 36,061		$ 36,431
Repurchases of common stock (in shares)	(2,755,065)		(7,019,032)
Repurchases of common stock	$ (165,304)		$ (421,143)
Ending balance (in shares)	241,538,824		244,865,658
Balance at end of period	$ 14,492,329		$ 14,691,939
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	852,771,508		807,928,139
Issuance of common stock from stock plan transactions	9,229,779		7,319,057
Repurchases of common stock	(8,540,700)		(20,214,812)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	5,894,645		6,356,070
Balance at end of period	859,355,232		801,388,454
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	1,089,698,996		1,116,372,133
Repurchases of common stock	(88,986,759)		(158,183,629)
Dividends declared	(28,984,621)		(29,383,859)
Net income	113,450,870		89,505,801
Balance at end of period	1,085,178,486		1,018,310,446
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	6,851,052		(1,289,103)
Other comprehensive (loss) income	(4,896,385)		1,503,897
Balance at end of period	$ 1,954,667		$ 214,794

[1] The condensed consolidated balance sheet at December 31, 2020 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements.