Consolidated Statement of Shareholders' Investment (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		143,076,357			143,076,357
Beginning balance		$ 1,120,960,556		$ 1,027,118,521	$ 1,120,960,556	$ 1,027,118,521	$ 893,530,826
Issuance of common stock and the tax benefit of stock plan transactions					42,822,635	12,208,881	33,304,762
Repurchases of common stock						(33,716,725)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					17,427,233	16,433,837	14,197,557
Dividends declared ($.48, $.52, and $.56 per share for the years ended December 31, 2011, 2012 and 2013 respectively)					(80,941,342)	(74,699,306)	(68,808,152)
Net Income	69,881,000	45,431,000	39,599,000	46,345,000	222,929,949	168,586,840	164,668,228
Other comprehensive income					4,404,652	5,028,508	(9,774,700)
Ending balance (in shares)	145,578,023		143,076,357		145,578,023	143,076,357
Ending balance	1,327,603,683		1,120,960,556		1,327,603,683	1,120,960,556	1,027,118,521
Common Stocks
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		143,076,357		144,069,563	143,076,357	144,069,563	142,292,127
Beginning balance		8,584,581		8,644,174	8,584,581	8,644,174	8,537,528
Issuance of common stock and the tax benefit of stock plan transactions (in shares)					2,501,666	978,623	1,777,436
Issuance of common stock and the tax benefit of stock plan transactions					150,100	58,717	106,646
Repurchases of common stock (in shares)						(1,971,829)
Repurchases of common stock						(118,310)
Ending balance (in shares)	145,578,023		143,076,357		145,578,023	143,076,357	144,069,563
Ending balance	8,734,681		8,584,581		8,734,681	8,584,581	8,644,174
Additional Paid-In Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		418,766,010		395,229,891	418,766,010	395,229,891	347,834,218
Issuance of common stock and the tax benefit of stock plan transactions					42,672,535	12,150,164	33,198,116
Repurchases of common stock						(5,047,882)
Stock-based compensation expense related to stock options, employee stock purchases and restricted stock					17,427,233	16,433,837	14,197,557
Ending balance	478,865,778		418,766,010		478,865,778	418,766,010	395,229,891
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		676,039,254		610,702,253	676,039,254	610,702,253	514,842,177
Repurchases of common stock						(28,550,533)
Dividends declared ($.48, $.52, and $.56 per share for the years ended December 31, 2011, 2012 and 2013 respectively)					(80,941,342)	(74,699,306)	(68,808,152)
Net Income					222,929,949	168,586,840	164,668,228
Ending balance	818,027,861		676,039,254		818,027,861	676,039,254	610,702,253
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance		17,570,711		12,542,203	17,570,711	12,542,203	22,316,903
Other comprehensive income					4,404,652	5,028,508	(9,774,700)
Ending balance	$ 21,975,363		$ 17,570,711		$ 21,975,363	$ 17,570,711	$ 12,542,203