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, 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
Numerator (basic and diluted):
Net income		$ 300,357		$ 366,151		$ 353,073		$ 304,906		$ 302,315		$ 283,734		$ 282,821		$ 264,934		$ 1,324,487	$ 1,133,804	$ 1,037,691
Denominator:
Denominator for basic earnings per share - weighted-average shares																		81,406	88,426	95,447
Effect of stock options	[1]																	874	1,076	1,273
Denominator for diluted earnings per share - weighted-average shares and assumed conversion																		82,280	89,502	96,720
Earnings per share - basic		$ 3.76	[2]	$ 4.54	[2]	$ 4.32	[2]	$ 3.65	[2]	$ 3.56	[2]	$ 3.26	[2]	$ 3.14	[2]	$ 2.88	[2]	$ 16.27	$ 12.82	$ 10.87
Earnings per share - assuming dilution		$ 3.72	[2]	$ 4.50	[2]	$ 4.28	[2]	$ 3.61	[2]	$ 3.52	[2]	$ 3.22	[2]	$ 3.10	[2]	$ 2.83	[2]	$ 16.10	$ 12.67	$ 10.73
Antidilutive stock options	[1]																	567	715	332
Weighted-average exercise price per share of antidilutive stock options	[1]																	$ 268.55	$ 252.16	$ 265.77

[1] See Note 10 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.