Quarterly Financial Information (Unaudited) (Schedule of Quarterly Financial Information) (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,370,953		$ 1,279,731		$ 1,270,949		$ 1,234,351		$ 1,231,342		$ 1,176,112		$ 1,163,068		$ 1,153,547		$ 5,155,984	$ 4,724,069	$ 4,431,178
Net income	$ 117,926		$ 77,308		$ 83,082		$ 116,487		$ 126,851		$ 93,619		$ 110,207		$ 118,610		$ 394,803	$ 449,287	$ 309,057
Net income per share
Basic	$ 0.86	[1]	$ 0.56	[1]	$ 0.59	[1]	$ 0.83	[1]	$ 0.88	[1]	$ 0.64	[1]	$ 0.73	[1]	$ 0.77	[1]	$ 2.83	$ 3.02	$ 1.93
Diluted	$ 0.82	[1]	$ 0.53	[1]	$ 0.56	[1]	$ 0.79	[1]	$ 0.85	[1]	$ 0.61	[1]	$ 0.70	[1]	$ 0.74	[1]	$ 2.71	$ 2.90	$ 1.86

[1] Net income per share (“EPS”) in each quarter is computed using the weighted-average number of shares outstanding during that quarter, while EPS for the full year is computed using the weighted-average number of shares outstanding during the year. Thus, the sum of the four quarters EPS does not necessarily equal the full-year EPS.