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, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator (basic and diluted):
Net income		$ 218,576		$ 266,268		$ 233,508		$ 212,864		$ 181,678		$ 216,997		$ 205,647		$ 173,860		$ 931,216	$ 778,182	$ 670,292
Denominator:
Denominator for basic earnings per share - weighted-average shares																		99,965	104,262	109,244
Effect of stock options	[1]																	1,549	1,779	1,857
Denominator for diluted earnings per share - weighted-average shares and assumed conversion																		101,514	106,041	111,101
Earnings per share - basic		$ 2.22	[2]	$ 2.68	[2]	$ 2.32	[2]	$ 2.09	[2]	$ 1.79	[2]	$ 2.10	[2]	$ 1.94	[2]	$ 1.64	[2]	$ 9.32	$ 7.46	$ 6.14
Earnings per share - assuming dilution		$ 2.19	[2]	$ 2.64	[2]	$ 2.29	[2]	$ 2.06	[2]	$ 1.76	[2]	$ 2.06	[2]	$ 1.91	[2]	$ 1.61	[2]	$ 9.17	$ 7.34	$ 6.03
Antidilutive stock options	[1]																	245	363	498
Weighted-average exercise price	[1]																	$ 216.29	$ 151.65	$ 103.80

[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.