Unaudited Condensed Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended
	Mar. 31, 2019		Mar. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 1,861,751,660	[1]	$ 2,049,518,261
Issuance of common stock and the tax benefit of stock plan transactions	$ 10,514,554		36,267,047
Repurchases of common stock (in shares)	(4,700,000)
Repurchases of common stock	$ (96,267,229)		(202,572,378)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	4,862,947		3,977,471
Impact of ASU 2016-01 adoption			0
Dividends declared	(29,392,200)		(30,123,981)
Net income	104,280,411		111,248,743
Other comprehensive income	1,324,604		1,662,333
Ending balance	$ 1,857,074,747		$ 1,969,977,496
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	259,328,613		280,281,321
Beginning balance	$ 15,559,717		$ 16,816,879
Issuance of common stock and the tax benefit of stock plan transactions (in shares)	980,670		2,904,555
Issuance of common stock and the tax benefit of stock plan transactions	$ 58,840		$ 174,274
Repurchases of common stock (in shares)	(4,724,938)		(9,331,499)
Repurchases of common stock	$ (283,497)		$ (559,890)
Ending balance (in shares)	255,584,345		273,854,377
Ending balance	$ 15,335,060		$ 16,431,263
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	745,324,144		723,510,672
Issuance of common stock and the tax benefit of stock plan transactions	10,455,714		36,092,773
Repurchases of common stock	(12,001,344)		(21,089,188)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	4,862,947		3,977,471
Ending balance	748,641,461		742,491,728
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,102,468,137		1,301,997,327
Repurchases of common stock	(83,982,388)		(180,923,300)
Impact of ASU 2016-01 adoption			6,642,727
Dividends declared	(29,392,200)		(30,123,981)
Net income	104,280,411		111,248,743
Ending balance	1,093,373,960		1,208,841,516
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(1,600,338)		7,193,383
Impact of ASU 2016-01 adoption			(6,642,727)
Other comprehensive income	1,324,604		1,662,333
Ending balance	$ (275,734)		$ 2,212,989

[1] The condensed consolidated balance sheet at December 31, 2018 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.