Consolidated Statement Of Shareholders' Equity (USD $)	Common Stocks [Member]	Additional Paid-In Capital [Member]	Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Beginning balance at Dec. 31, 2008	$ 8,258,010	$ 253,821,363		$ 434,975,514	$ 1,541,435	$ 698,596,322
Beginning balance, shares at Dec. 31, 2008	137,633,502
Issuance of common stock and the tax benefit of stock plan transactions, shares	705,883
Issuance of common stock and the tax benefit of stock plan transactions	42,353	7,445,542				7,487,895
Stock-based compensation expense related to employee stock options, employee stock purchases and restricted stock		9,084,891				9,084,891
Dividends declared				(60,675,357)		(60,675,357)
Comprehensive income:
Net income			64,637,085	64,637,085		64,637,085
Other comprehensive income (loss):
Foreign currency translation adjustment			1,585,745
Unrealized gain (loss) on investments, net of tax			15,212,162
Other comprehensive income (loss)			16,797,907		16,797,907	16,797,907
Comprehensive income			81,434,992
Ending balance at Dec. 31, 2009	8,300,363	270,351,796		438,937,242	18,339,342	735,928,743
Ending balance, shares at Dec. 31, 2009	138,339,385
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	237,165	66,830,226				67,067,391
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)
Comprehensive income:
Net income			137,733,969	137,733,969		137,733,969
Other comprehensive income (loss):
Foreign currency translation adjustment			(2,176,851)
Unrealized gain (loss) on investments, net of tax			6,154,412
Other comprehensive income (loss)			3,977,561		3,977,561	3,977,561
Comprehensive income			141,711,530
Ending balance at Dec. 31, 2010	8,537,528	347,834,218		514,842,177	22,316,903	893,530,826
Ending balance, shares at Dec. 31, 2010	142,292,127					142,292,127
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	106,646	33,198,116				33,304,762
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)
Comprehensive income:
Net income			164,668,228	164,668,228		164,668,228
Other comprehensive income (loss):
Foreign currency translation adjustment			1,563,054
Unrealized gain (loss) on investments, net of tax			(11,337,754)
Other comprehensive income (loss)			(9,774,700)		(9,774,700)	(9,774,700)
Comprehensive income			154,893,528
Ending balance at Dec. 31, 2011	$ 8,644,174	$ 395,229,891		$ 610,702,253	$ 12,542,203	$ 1,027,118,521
Ending balance, shares at Dec. 31, 2011	144,069,563					144,069,563