Earnings Per Share (Schedule of Earnings Per Share, Basic And Diluted) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerators:
Numerator for both basic and diluted EPS, net income	$ 111,335,817	$ 90,230,335	$ 331,608,542	$ 276,323,023
Denominators:
Denominator for basic EPS, weighted-average shares outstanding (in shares)	265,607,128	284,661,920	270,366,996	285,915,565
Potentially dilutive shares resulting from stock plans (in shares)	1,988,014	2,359,030	2,366,506	3,145,936
Denominator for diluted EPS (in shares)	267,595,142	287,020,950	272,733,502	289,061,501
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be anti-dilutive (in shares)	462,631	807,013	24,590	2,847,247