Unaudited Condensed Consolidated Statements of Shareholders' Investment - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 1,822,607,413	$ 1,904,334,235	$ 1,938,087,820	[1]	$ 1,861,751,660
Issuance of common stock from stock plan transactions	7,966,489	26,287,061	$ 23,677,181		68,221,875
Issuance of common stock related to acquisitions (in shares)			100,000
Issuance of common stock related to acquisitions			$ 3,559,229
Repurchases of common stock (in shares)			(8,200,000)
Repurchases of common stock	(31,660,862)	(96,625,003)	$ (210,480,446)		(262,763,375)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	7,306,543	5,788,286	19,093,196		15,927,434
Dividends declared	(29,407,702)	(29,092,442)	(88,285,214)		(87,784,052)
Net income	117,092,602	111,897,852	204,224,370		325,136,888
Other comprehensive income (loss)	1,707,816	(1,261,726)	5,736,163		837,833
Balance at end of period	$ 1,895,612,299	$ 1,921,328,263	$ 1,895,612,299		$ 1,921,328,263
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	245,775,488	254,779,119	251,277,515		259,328,613
Balance at beginning of period	$ 14,746,529	$ 15,286,747	$ 15,076,651		$ 15,559,717
Issuance of common stock from stock plan transactions (in shares)	461,983	1,771,463	1,815,270		5,021,616
Issuance of common stock from stock plan transactions	$ 27,719	$ 106,287	$ 108,917		$ 301,297
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)	(1,175,756)	(3,569,577)	(8,194,788)		(11,369,224)
Repurchases of common stock	$ (70,545)	$ (214,174)	$ (491,688)		$ (682,154)
Ending balance (in shares)	245,061,715	252,981,005	245,061,715		252,981,005
Balance at end of period	$ 14,703,703	$ 15,178,860	$ 14,703,703		$ 15,178,860
Additional  Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	818,678,880	777,238,253	807,928,139		745,324,144
Issuance of common stock from stock plan transactions	7,938,770	26,180,774	23,568,264		67,920,578
Issuance of common stock related to acquisitions			3,549,406
Repurchases of common stock	(3,515,510)	(9,744,945)	(23,730,322)		(29,709,788)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock	7,306,543	5,788,286	19,093,196		15,927,434
Balance at end of period	830,408,683	799,462,368	830,408,683		799,462,368
Retained  Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	986,442,760	1,111,310,014	1,116,372,133		1,102,468,137
Repurchases of common stock	(28,074,807)	(86,665,884)	(186,258,436)		(232,371,433)
Dividends declared	(29,407,702)	(29,092,442)	(88,285,214)		(87,784,052)
Net income	117,092,602	111,897,852	204,224,370		325,136,888
Balance at end of period	1,046,052,853	1,107,449,540	1,046,052,853		1,107,449,540
Accumulated  Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	2,739,244	499,221	(1,289,103)		(1,600,338)
Other comprehensive income (loss)	1,707,816	(1,261,726)	5,736,163		837,833
Balance at end of period	$ 4,447,060	$ (762,505)	$ 4,447,060		$ (762,505)

[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.