Summary of Significant Accounting and Reporting Policies (Schedule of Earnings Per Share, Basic And Diluted) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerators:
Numerator for both basic and diluted EPS, net income	$ 130,469,000	$ 90,230,000	$ 88,536,000	$ 97,557,000	$ 88,761,000	$ 92,065,000	$ 86,485,000	$ 80,280,000	$ 406,791,922	$ 347,591,276	$ 318,469,859
Denominators:
Denominator for basic EPS, weighted-average common shares outstanding (in shares)									285,864,997	288,433,772	293,096,212
Potentially dilutive shares resulting from stock option plans (in shares)									2,361,092	2,638,544	3,141,687
Denominator for diluted EPS (in shares)									288,226,089	291,072,316	296,237,899