Consolidated Statement of Shareholders' Investment - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		295,247,958			295,247,958
Beginning balance		$ 1,571,412,445		$ 1,327,603,683	$ 1,571,412,445	$ 1,327,603,683	$ 1,120,960,556
Issuance of common stock and the tax benefit of stock plan transactions					33,006,225	65,471,344	42,822,635
Repurchases of common stock					(111,228,825)	(30,010,882)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,430,991	21,276,991	17,427,233
Dividends declared					(98,130,083)	(90,870,922)	(80,941,342)
Net Income	$ 88,396,000	$ 77,183,000	$ 70,975,000	$ 68,566,000	318,469,859	288,604,579	222,929,949
Other comprehensive income					$ (12,443,851)	$ (10,662,348)	4,404,652
Ending balance (in shares)	291,338,011		295,247,958		291,338,011	295,247,958
Ending balance	$ 1,722,516,761		$ 1,571,412,445		$ 1,722,516,761	$ 1,571,412,445	$ 1,327,603,683
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		295,247,958		291,156,046	295,247,958	291,156,046	286,152,714
Beginning balance		$ 17,714,877		$ 17,469,362	$ 17,714,877	$ 17,469,362	$ 17,169,162
Issuance of common stock and the tax benefit of stock plan transactions (in shares)					2,740,626	5,889,392	5,003,332
Issuance of common stock and the tax benefit of stock plan transactions					$ 164,438	$ 353,362	$ 300,200
Repurchases of common stock (in shares)					(6,650,573)	(1,797,480)
Repurchases of common stock					$ (399,034)	$ (107,847)
Ending balance (in shares)	291,338,011		295,247,958		291,338,011	295,247,958	291,156,046
Ending balance	$ 17,480,281		$ 17,714,877		$ 17,480,281	$ 17,714,877	$ 17,469,362
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		553,836,483		470,131,097	553,836,483	470,131,097	410,181,429
Issuance of common stock and the tax benefit of stock plan transactions					32,841,787	65,117,982	42,522,435
Repurchases of common stock					(11,326,566)	(2,689,587)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,430,991	21,276,991	17,427,233
Ending balance	596,782,695		553,836,483		596,782,695	553,836,483	470,131,097
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		988,548,070		818,027,861	988,548,070	818,027,861	676,039,254
Repurchases of common stock					(99,503,225)	(27,213,448)
Dividends declared					(98,130,083)	(90,870,922)	(80,941,342)
Net Income					318,469,859	288,604,579	222,929,949
Ending balance	1,109,384,621		988,548,070		1,109,384,621	988,548,070	818,027,861
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		$ 11,313,015		$ 21,975,363	11,313,015	21,975,363	17,570,711
Other comprehensive income					(12,443,851)	(10,662,348)	4,404,652
Ending balance	$ (1,130,836)		$ 11,313,015		$ (1,130,836)	$ 11,313,015	$ 21,975,363