Earnings Per Share (Schedule of Earnings Per Share, Basic And Diluted) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Earnings Per Share
Numerator for both basic and diluted EPS, net income	$ 38,472,169	$ 34,057,330	$ 80,805,051	$ 66,519,460
Denominator for basic EPS, weighted-average shares outstanding	142,279,210	139,124,771	142,071,203	138,697,007
Potentially dilutive shares resulting from stock plans	1,917,784	1,421,509	1,992,412	1,365,635
Denominator for diluted EPS	144,196,994	140,546,280	144,063,615	140,062,642
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be antidilutive	1,078,512	285,998	307,231	301,038