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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Numerator (basic and diluted):
Net income	$ 132,802	$ 159,332	$ 146,120	$ 147,492	$ 122,988	$ 148,439	$ 133,772	$ 102,474	$ 585,746		$ 507,673		$ 419,373
Denominator:
Denominator for basic earnings per share - weighted-average shares									121,182		134,667		138,654
Effect of stock options									2,132	[1]	2,316	[1]	2,348	[1]
Effect of exhangeable notes									0		0		990
Denominator for diluted earnings per share - weighted-average shares and assumed conversion									123,314		136,983		141,992
Earnings per share - basic	$ 1.16	$ 1.34	$ 1.17	$ 1.16	$ 0.96	$ 1.12	$ 0.97	$ 0.73	$ 4.83		$ 3.77		$ 3.02
Earnings per share - assuming dilution	$ 1.14	$ 1.32	$ 1.15	$ 1.14	$ 0.94	$ 1.10	$ 0.96	$ 0.72	$ 4.75		$ 3.71		$ 2.95

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