Consolidated Statement Of Shareholders' Equity Investment (USD $)	Total	Common Stocks [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2009	$ 735,928,743	$ 8,300,363	$ 270,351,796	$ 438,937,242	$ 18,339,342
Beginning balance, shares at Dec. 31, 2009		138,339,385
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock and the tax benefit of stock plan transactions, shares		3,952,742
Issuance of common stock and the tax benefit of stock plan transactions	67,067,391	237,165	66,830,226
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	10,652,196		10,652,196
Dividends declared	(61,829,034)			(61,829,034)
Net Income	137,733,969			137,733,969
Other comprehensive income (loss)	3,977,561				3,977,561
Ending balance at Dec. 31, 2010	893,530,826	8,537,528	347,834,218	514,842,177	22,316,903
Ending balance, shares at Dec. 31, 2010		142,292,127
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock and the tax benefit of stock plan transactions, shares		1,777,436
Issuance of common stock and the tax benefit of stock plan transactions	33,304,762	106,646	33,198,116
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	14,197,557		14,197,557
Dividends declared	(68,808,152)			(68,808,152)
Net Income	164,668,228			164,668,228
Other comprehensive income (loss)	(9,774,700)				(9,774,700)
Ending balance at Dec. 31, 2011	1,027,118,521	8,644,174	395,229,891	610,702,253	12,542,203
Ending balance, shares at Dec. 31, 2011	144,069,563	144,069,563
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock and the tax benefit of stock plan transactions, shares		978,623
Issuance of common stock and the tax benefit of stock plan transactions	12,208,881	58,717	12,150,164
Repurchases of common stock, shares		(1,971,829)
Repurchases of common stock	(33,716,725)	(118,310)	(5,047,882)	(28,550,533)
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock	16,433,837		16,433,837
Dividends declared	(74,699,306)			(74,699,306)
Net Income	168,586,840			168,586,840
Other comprehensive income (loss)	5,028,508				5,028,508
Ending balance at Dec. 31, 2012	$ 1,120,960,556	$ 8,584,581	$ 418,766,010	$ 676,039,254	$ 17,570,711
Ending balance, shares at Dec. 31, 2012	143,076,357	143,076,357