Earnings Per Share (Schedule of Earnings Per Share, Basic And Diluted) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerators:
Numerator for both basic and diluted EPS, net income	$ 109,023,982	$ 88,535,947	$ 220,272,725	$ 186,092,688
Denominators:
Denominator for basic EPS, weighted-average shares outstanding (in shares)	271,747,049	286,722,482	273,085,191	286,956,367
Potentially dilutive shares resulting from stock plans (in shares)	2,375,449	3,120,420	2,575,227	3,581,875
Denominator for diluted EPS (in shares)	274,122,498	289,842,902	275,660,418	290,538,242
Shares related to stock plans not included in diluted average common shares outstanding because their effect would be anti-dilutive (in shares)	24,231	873,937	12,182	109,597