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, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator (basic and diluted):
Net income		$ 246,030		$ 278,493		$ 257,794		$ 255,374		$ 218,576		$ 266,268		$ 233,508		$ 212,864		$ 1,037,691	$ 931,216	$ 778,182
Denominator:
Denominator for basic earnings per share - weighted-average shares																		95,447	99,965	104,262
Effect of stock options	[1]																	1,273	1,549	1,779
Denominator for diluted earnings per share - weighted-average shares and assumed conversion																		96,720	101,514	106,041
Earnings per share - basic		$ 2.62	[2]	$ 2.93	[2]	$ 2.69	[2]	$ 2.63	[2]	$ 2.22	[2]	$ 2.68	[2]	$ 2.32	[2]	$ 2.09	[2]	$ 10.87	$ 9.32	$ 7.46
Earnings per share - assuming dilution		$ 2.59	[2]	$ 2.90	[2]	$ 2.65	[2]	$ 2.59	[2]	$ 2.19	[2]	$ 2.64	[2]	$ 2.29	[2]	$ 2.06	[2]	$ 10.73	$ 9.17	$ 7.34
Antidilutive stock options	[1]																	332	245	363
Weighted-average exercise price	[1]																	$ 265.77	$ 216.29	$ 151.65

[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.