Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Numerator (basic and diluted):
Net income	$ 181,678		$ 216,997		$ 205,647		$ 173,860		$ 152,347		$ 186,489		$ 177,127		$ 154,329		$ 778,182		$ 670,292		$ 585,746
Denominator:
Denominator for basic earnings per share - weighted-average shares																	104,262		109,244		121,182
Effect of stock options																	1,779	[1]	1,857	[1]	2,132	[1]
Denominator for diluted earnings per share - weighted-average shares and assumed conversion																	106,041		111,101		123,314
Earnings per share - basic	$ 1.79	[2]	$ 2.10	[2]	$ 1.94	[2]	$ 1.64	[2]	$ 1.43	[2]	$ 1.72	[2]	$ 1.61	[2]	$ 1.38	[2]	$ 7.46		$ 6.14		$ 4.83
Earnings per share - assuming dilution	$ 1.76	[2]	$ 2.06	[2]	$ 1.91	[2]	$ 1.61	[2]	$ 1.40	[2]	$ 1.69	[2]	$ 1.58	[2]	$ 1.36	[2]	$ 7.34		$ 6.03		$ 4.75
Antidilutive stock options																	363	[1]	498	[1]	1,816	[1]
Weighted-average exercise price																	$ 151.65	[1]	$ 103.80	[1]	$ 87.88	[1]

[1] See Note 9 for further discussion on 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.