Consolidated Statement of Shareholders' Investment (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		291,156,046			291,156,046
Beginning balance		$ 1,327,603,683		$ 1,120,960,556	$ 1,327,603,683	$ 1,120,960,556	$ 1,027,118,521
Issuance of common stock and the tax benefit of stock plan transactions					65,471,344	42,822,635	12,208,881
Repurchases of common stock					(30,010,882)		(33,716,725)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,276,991	17,427,233	16,433,837
Dividends declared ($.26, $.28, and $.31 per share for the years ended December 31, 2012, 2013 and 2014 respectively)					(90,870,922)	(80,941,342)	(74,699,306)
Net Income	70,975,000	68,566,000	69,881,000	45,431,000	288,604,579	222,929,949	168,586,840
Other comprehensive income					(10,662,348)	4,404,652	5,028,508
Ending balance (in shares)	295,247,958		291,156,046		295,247,958	291,156,046
Ending balance	1,571,412,445		1,327,603,683		1,571,412,445	1,327,603,683	1,120,960,556
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		291,156,046		286,152,714	291,156,046	286,152,714	288,139,126
Beginning balance		17,469,362		17,169,162	17,469,362	17,169,162	17,288,348
Issuance of common stock and the tax benefit of stock plan transactions (in shares)					5,889,392	5,003,332	1,957,246
Issuance of common stock and the tax benefit of stock plan transactions					353,362	300,200	117,434
Repurchases of common stock (in shares)					(1,797,480)		(3,943,658)
Repurchases of common stock					(107,847)		(236,620)
Ending balance (in shares)	295,247,958		291,156,046		295,247,958	291,156,046	286,152,714
Ending balance	17,714,877		17,469,362		17,714,877	17,469,362	17,169,162
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		470,131,097		410,181,429	470,131,097	410,181,429	386,585,717
Issuance of common stock and the tax benefit of stock plan transactions					65,117,982	42,522,435	12,091,447
Repurchases of common stock					(2,689,587)		(4,929,572)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					21,276,991	17,427,233	16,433,837
Ending balance	553,836,483		470,131,097		553,836,483	470,131,097	410,181,429
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		818,027,861		676,039,254	818,027,861	676,039,254	610,702,253
Repurchases of common stock					(27,213,448)		(28,550,533)
Dividends declared ($.26, $.28, and $.31 per share for the years ended December 31, 2012, 2013 and 2014 respectively)					(90,870,922)	(80,941,342)	(74,699,306)
Net Income					288,604,579	222,929,949	168,586,840
Ending balance	988,548,070		818,027,861		988,548,070	818,027,861	676,039,254
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		21,975,363		17,570,711	21,975,363	17,570,711	12,542,203
Other comprehensive income					(10,662,348)	4,404,652	5,028,508
Ending balance	$ 11,313,015		$ 21,975,363		$ 11,313,015	$ 21,975,363	$ 17,570,711