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, 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
Numerator (basic and diluted):
Net income		$ 302,315		$ 283,734		$ 282,821		$ 264,934		$ 246,030		$ 278,493		$ 257,794		$ 255,374		$ 1,133,804	$ 1,037,691	$ 931,216
Denominator:
Denominator for basic earnings per share - weighted-average shares																		88,426	95,447	99,965
Effect of stock options	[1]																	1,076	1,273	1,549
Denominator for diluted earnings per share - weighted-average shares and assumed conversion																		89,502	96,720	101,514
Earnings per share - basic		$ 3.56	[2]	$ 3.26	[2]	$ 3.14	[2]	$ 2.88	[2]	$ 2.62	[2]	$ 2.93	[2]	$ 2.69	[2]	$ 2.63	[2]	$ 12.82	$ 10.87	$ 9.32
Earnings per share - assuming dilution		$ 3.52	[2]	$ 3.22	[2]	$ 3.10	[2]	$ 2.83	[2]	$ 2.59	[2]	$ 2.90	[2]	$ 2.65	[2]	$ 2.59	[2]	$ 12.67	$ 10.73	$ 9.17
Antidilutive stock options	[1]																	715	332	245
Weighted-average exercise price per share of antidilutive stock options	[1]																	$ 252.16	$ 265.77	$ 216.29

[1] See Note 9 for further information concerning the terms of the Company’s share-based compensation plans.

[2] Earnings per share amounts are computed independently for each quarter and annual period. The quarterly earnings per share amounts may not sum to equal the full-year earnings per share amount.