Consolidated Statement of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		291,338,011			291,338,011
Beginning balance		$ 1,722,516,761		$ 1,571,412,445	$ 1,722,516,761	$ 1,571,412,445	$ 1,327,603,683
Issuance of common stock and the tax benefit of stock plan transactions					86,893,439	33,006,225	65,471,344
Repurchases of common stock					(163,361,221)	(111,228,825)	(30,010,882)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					19,192,699	21,430,991	21,276,991
Dividends declared					(102,268,251)	(98,130,083)	(90,870,922)
Net Income	$ 88,761,000	$ 80,280,000	$ 88,396,000	$ 77,183,000	347,591,276	318,469,859	288,604,579
Other comprehensive income					$ (140,469)	$ (12,443,851)	(10,662,348)
Ending balance (in shares)	287,737,516		291,338,011		287,737,516	291,338,011
Ending balance	$ 1,910,424,234		$ 1,722,516,761		$ 1,910,424,234	$ 1,722,516,761	$ 1,571,412,445
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		291,338,011		295,247,958	291,338,011	295,247,958	291,156,046
Beginning balance		$ 17,480,281		$ 17,714,877	$ 17,480,281	$ 17,714,877	$ 17,469,362
Issuance of common stock and the tax benefit of stock plan transactions (in shares)					6,705,632	2,740,626	5,889,392
Issuance of common stock and the tax benefit of stock plan transactions					$ 402,338	$ 164,438	$ 353,362
Repurchases of common stock (in shares)					(10,306,127)	(6,650,573)	(1,797,480)
Repurchases of common stock					$ (618,368)	$ (399,034)	$ (107,847)
Ending balance (in shares)	287,737,516		291,338,011		287,737,516	291,338,011	295,247,958
Ending balance	$ 17,264,251		$ 17,480,281		$ 17,264,251	$ 17,480,281	$ 17,714,877
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		596,782,695		553,836,483	596,782,695	553,836,483	470,131,097
Issuance of common stock and the tax benefit of stock plan transactions					86,491,101	32,841,787	65,117,982
Repurchases of common stock					(19,020,032)	(11,326,566)	(2,689,587)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					19,192,699	21,430,991	21,276,991
Ending balance	683,446,463		596,782,695		683,446,463	596,782,695	553,836,483
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		1,109,384,621		988,548,070	1,109,384,621	988,548,070	818,027,861
Repurchases of common stock					(143,722,821)	(99,503,225)	(27,213,448)
Dividends declared					(102,268,251)	(98,130,083)	(90,870,922)
Net Income					347,591,276	318,469,859	288,604,579
Ending balance	1,210,984,825		1,109,384,621		1,210,984,825	1,109,384,621	988,548,070
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ (1,130,836)		$ 11,313,015	(1,130,836)	11,313,015	21,975,363
Other comprehensive income					(140,469)	(12,443,851)	(10,662,348)
Ending balance	$ (1,271,305)		$ (1,130,836)		$ (1,271,305)	$ (1,130,836)	$ 11,313,015