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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerators:
Numerator for both basic and diluted EPS, net income	$ 106,275,000	$ 111,336,000	$ 109,024,000	$ 111,249,000	$ 130,469,000	$ 90,230,000	$ 88,536,000	$ 97,557,000	$ 437,883,097	$ 406,791,922	$ 347,591,276
Denominators:
Denominator for basic EPS, weighted-average common shares outstanding (in shares)									267,794,786	285,864,997	288,433,772
Potentially dilutive shares resulting from stock option plans (in shares)									2,082,563	2,361,092	2,638,544
Denominator for diluted EPS (in shares)									269,877,349	288,226,089	291,072,316