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, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Numerator (basic and diluted):
Net income	$ 122,988	$ 148,439	$ 133,772	$ 102,474	$ 105,760	$ 116,542	$ 99,595	$ 97,476	$ 507,673		$ 419,373		$ 307,498
Denominator:
Denominator for basic earnings per share - weighted-average shares									134,667		138,654		136,230
Effect of stock options									2,316	[1]	2,348	[1]	1,651	[1]
Effect of exchangeable notes									0		990		1
Denominator for diluted earnings per share - weighted-average shares and assumed conversion									136,983		141,992		137,882
Earnings per share - basic	$ 0.96	$ 1.12	$ 0.97	$ 0.73	$ 0.76	$ 0.84	$ 0.72	$ 0.71	$ 3.77		$ 3.02		$ 2.26
Earnings per share - assuming dilution	$ 0.94	$ 1.10	$ 0.96	$ 0.72	$ 0.74	$ 0.82	$ 0.71	$ 0.70	$ 3.71		$ 2.95		$ 2.23

[1] See Note 10 for further discussion on the terms of the Company's share-based compensation plans.