Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator (basic and diluted):
Net income	$ 324,916	$ 391,293	$ 353,681	$ 321,152	$ 300,357	$ 366,151	$ 353,073	$ 304,906	$ 1,391,042	$ 1,324,487	$ 1,133,804
Denominator:
Denominator for basic earnings per share - weighted-average shares									76,985	81,406	88,426
Effect of stock options									803	874	1,076
Denominator for diluted earnings per share - weighted-average shares and assumed conversion									77,788	82,280	89,502
Earnings per share - basic	$ 4.29	$ 5.14	$ 4.56	$ 4.09	$ 3.76	$ 4.54	$ 4.32	$ 3.65	$ 18.07	$ 16.27	$ 12.82
Earnings per share - assuming dilution	$ 4.25	$ 5.08	$ 4.51	$ 4.05	$ 3.72	$ 4.50	$ 4.28	$ 3.61	$ 17.88	$ 16.10	$ 12.67
Antidilutive stock options									229	567	715
Weighted-average exercise price per share of antidilutive stock options									$ 368.11	$ 268.55	$ 252.16