Unaudited Condensed Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 1,938,087,820	[1]	$ 1,861,751,660
Issuance of common stock from stock plan transactions	$ 7,355,488		10,514,554
Repurchases of common stock (in shares)	(7,000,000.0)
Repurchases of common stock	$ (178,819,584)		(96,267,228)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	6,356,070		4,862,947
Dividends declared	(29,383,859)		(29,392,200)
Net Income	89,505,801		104,280,411
Other comprehensive loss	1,503,897		1,324,604
Balance at end of period	$ 1,834,605,633		$ 1,857,074,748
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	251,277,515		259,328,613
Balance at beginning of period	$ 15,076,651		$ 15,559,717
Issuance of common stock from stock plan transactions (in shares)	607,175		980,670
Issuance of common stock from stock plan transactions	$ 36,431		$ 58,840
Repurchases of common stock (in shares)	(7,019,032)		(4,724,938)
Repurchases of common stock	$ (421,143)		$ (283,496)
Ending balance (in shares)	244,865,658		255,584,345
Balance at end of period	$ 14,691,939		$ 15,335,061
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	807,928,139		745,324,144
Issuance of common stock from stock plan transactions	7,319,057		10,455,714
Repurchases of common stock	(20,214,812)		(12,001,344)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	6,356,070		4,862,947
Balance at end of period	801,388,454		748,641,461
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	1,116,372,133		1,102,468,137
Repurchases of common stock	(158,183,629)		(83,982,388)
Dividends declared	(29,383,859)		(29,392,200)
Net Income	89,505,801		104,280,411
Balance at end of period	1,018,310,446		1,093,373,960
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(1,289,103)		(1,600,338)
Other comprehensive loss	1,503,897		1,324,604
Balance at end of period	$ 214,794		$ (275,734)

[1] The condensed consolidated balance sheet at December 31, 2019 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.