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, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,696,679		$ 1,634,124		$ 1,570,962		$ 1,506,769		$ 1,608,601		$ 1,420,501		$ 1,415,747		$ 1,378,705		$ 6,408,534	$ 5,823,554	$ 5,155,984
Net income	$ 130,379		$ 136,974		$ 115,957		$ 116,615		$ 165,489		$ 100,947		$ 108,838		$ 135,318		$ 499,925	$ 510,592	$ 391,211
Net income per share
Basic (dollars per share)	$ 0.97	[1]	$ 1.01	[1]	$ 0.85	[1]	$ 0.86	[1]	$ 1.22	[1]	$ 0.74	[1]	$ 0.79	[1]	$ 0.98	[1]	$ 3.69	$ 3.72	$ 2.80
Diluted (dollars per share)	$ 0.93	[1]	$ 0.97	[1]	$ 0.82	[1]	$ 0.83	[1]	$ 1.17	[1]	$ 0.71	[1]	$ 0.76	[1]	$ 0.94	[1]	$ 3.55	$ 3.56	$ 2.69

[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.