Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		251,277,515			251,277,515
Beginning balance		$ 1,938,087,820		$ 1,861,751,660	$ 1,938,087,820	$ 1,861,751,660	$ 2,049,518,261
Issuance of common stock from stock plan transactions					41,803,640	77,821,151	66,837,820
Issuance of common stock related to acquisitions					3,559,229
Repurchases of common stock					(288,480,506)	(331,471,392)	(591,577,851)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					30,797,327	21,671,192	18,305,981
Accounting standards update								us-gaap:AccountingStandardsUpdate201601Member
Dividends declared					(117,528,158)	(116,679,965)	(117,064,654)
Net income	$ 143,339,000	$ 89,506,000	$ 99,547,000	$ 104,280,000	347,563,621	424,683,939	437,883,097
Other comprehensive income					$ 8,140,155	$ 311,235	(2,150,994)
Ending balance (in shares)	243,692,869		251,277,515		243,692,869	251,277,515
Ending balance	$ 1,963,943,128		$ 1,938,087,820		$ 1,963,943,128	$ 1,938,087,820	1,861,751,660	$ 2,049,518,261
Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance							$ 0
Ending balance								$ 0
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		251,277,515		259,328,613	251,277,515	259,328,613	280,281,321
Beginning balance		$ 15,076,651		$ 15,559,717	$ 15,076,651	$ 15,559,717	$ 16,816,879
Issuance of common stock from stock plan transactions (in shares)					2,897,689	5,724,840	5,496,659
Issuance of common stock from stock plan transactions					$ 173,861	$ 343,490	$ 329,801
Issuance of common stock related to acquisitions (in shares)					163,718
Issuance of common stock related to acquisitions					$ 9,823
Repurchases of common stock (in shares)					(10,646,053)	(13,775,938)	(26,449,367)
Repurchases of common stock					$ (638,763)	$ (826,556)	$ (1,586,963)
Ending balance (in shares)	243,692,869		251,277,515		243,692,869	251,277,515	259,328,613	280,281,321
Ending balance	$ 14,621,572		$ 15,076,651		$ 14,621,572	$ 15,076,651	$ 15,559,717	$ 16,816,879
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		807,928,139		745,324,144	807,928,139	745,324,144	723,510,672
Issuance of common stock from stock plan transactions					41,629,779	77,477,661	66,508,019
Issuance of common stock related to acquisitions					3,549,406
Repurchases of common stock					(31,133,143)	(36,544,858)	(63,000,528)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					30,797,327	21,671,192	18,305,981
Ending balance	852,771,508		807,928,139		852,771,508	807,928,139	745,324,144	723,510,672
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,116,372,133		1,102,468,137	1,116,372,133	1,102,468,137	1,301,997,327
Repurchases of common stock					(256,708,600)	(294,099,978)	(526,990,360)
Dividends declared					(117,528,158)	(116,679,965)	(117,064,654)
Net income					347,563,621	424,683,939	437,883,097
Ending balance	1,089,698,996		1,116,372,133		1,089,698,996	1,116,372,133	1,102,468,137	1,301,997,327
Retained  Earnings | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance							6,642,727
Ending balance								6,642,727
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,289,103)		$ (1,600,338)	(1,289,103)	(1,600,338)	7,193,383
Other comprehensive income					8,140,155	311,235	(2,150,994)
Ending balance	$ 6,851,052		$ (1,289,103)		$ 6,851,052	$ (1,289,103)	(1,600,338)	7,193,383
Accumulated  Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance							$ (6,642,727)
Ending balance								$ (6,642,727)