Consolidated Statement of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		287,737,516			287,737,516
Beginning balance		$ 1,910,424,234		$ 1,722,516,761	$ 1,910,424,234	$ 1,722,516,761	$ 1,571,412,445
Issuance of common stock and the tax benefit of stock plan transactions					47,770,467	86,893,439	33,006,225
Repurchases of common stock					(231,363,216)	(163,361,221)	(111,228,825)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					18,376,965	19,192,699	21,430,991
Dividends declared					(110,946,799)	(102,268,251)	(98,130,083)
Net Income	$ 130,469,000	$ 97,557,000	$ 88,761,000	$ 80,280,000	406,791,922	347,591,276	318,469,859
Other comprehensive income					$ 8,464,688	$ (140,469)	(12,443,851)
Ending balance (in shares)	280,281,321		287,737,516		280,281,321	287,737,516
Ending balance	$ 2,049,518,261		$ 1,910,424,234		$ 2,049,518,261	$ 1,910,424,234	$ 1,722,516,761
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		287,737,516		291,338,011	287,737,516	291,338,011	295,247,958
Beginning balance		$ 17,264,251		$ 17,480,281	$ 17,264,251	$ 17,480,281	$ 17,714,877
Issuance of common stock and the tax benefit of stock plan transactions (in shares)					4,498,729	6,705,632	2,740,626
Issuance of common stock and the tax benefit of stock plan transactions					$ 269,923	$ 402,338	$ 164,438
Repurchases of common stock (in shares)					(11,954,924)	(10,306,127)	(6,650,573)
Repurchases of common stock					$ (717,295)	$ (618,368)	$ (399,034)
Ending balance (in shares)	280,281,321		287,737,516		280,281,321	287,737,516	291,338,011
Ending balance	$ 16,816,879		$ 17,264,251		$ 16,816,879	$ 17,264,251	$ 17,480,281
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		683,446,463		596,782,695	683,446,463	596,782,695	553,836,483
Issuance of common stock and the tax benefit of stock plan transactions					47,500,544	86,491,101	32,841,787
Repurchases of common stock					(25,813,300)	(19,020,032)	(11,326,566)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					18,376,965	19,192,699	21,430,991
Ending balance	723,510,672		683,446,463		723,510,672	683,446,463	596,782,695
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,210,984,825		1,109,384,621	1,210,984,825	1,109,384,621	988,548,070
Repurchases of common stock					(204,832,621)	(143,722,821)	(99,503,225)
Dividends declared					(110,946,799)	(102,268,251)	(98,130,083)
Net Income					406,791,922	347,591,276	318,469,859
Ending balance	1,301,997,327		1,210,984,825		1,301,997,327	1,210,984,825	1,109,384,621
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,271,305)		$ (1,130,836)	(1,271,305)	(1,130,836)	11,313,015
Other comprehensive income					8,464,688	(140,469)	(12,443,851)
Ending balance	$ 7,193,383		$ (1,271,305)		$ 7,193,383	$ (1,271,305)	$ (1,130,836)