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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator (basic and diluted):
Net income	$ 392,945	$ 527,252	$ 531,667	$ 300,438	$ 324,916	$ 391,293	$ 353,681	$ 321,152	$ 1,752,302	$ 1,391,042	$ 1,324,487
Denominator:
Denominator for basic earnings per share - weighted-average shares									73,817	76,985	81,406
Effect of stock options									645	803	874
Denominator for diluted earnings per share - weighted-average shares and assumed conversion									74,462	77,788	82,280
Earnings per share - basic	$ 5.45	$ 7.13	$ 7.16	$ 4.00	$ 4.29	$ 5.14	$ 4.56	$ 4.09	$ 23.74	$ 18.07	$ 16.27
Earnings per share - assuming dilution	$ 5.40	$ 7.07	$ 7.10	$ 3.97	$ 4.25	$ 5.08	$ 4.51	$ 4.05	$ 23.53	$ 17.88	$ 16.10
Antidilutive stock options									291	229	567
Weighted-average exercise price per share of antidilutive stock options									$ 393.42	$ 368.11	$ 268.55