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, 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,784,428	$ 1,840,605	$ 1,796,989	$ 1,706,906	$ 1,696,679		$ 1,634,124		$ 1,570,962		$ 1,506,769		$ 7,128,928	$ 6,408,534	$ 5,823,554
Net income	$ 110,711	$ 188,539	$ 179,961	$ 169,673	$ 130,379		$ 136,974		$ 115,957		$ 116,615		$ 648,884	$ 499,925	$ 510,592
Net income per share
Basic (dollars per share)	$ 0.87	$ 1.48	$ 1.41	$ 1.31	$ 0.97	[1]	$ 1.01	[1]	$ 0.85	[1]	$ 0.86	[1]	$ 5.07	$ 3.69	$ 3.72
Diluted (dollars per share)	$ 0.83	$ 1.42	$ 1.35	$ 1.25	$ 0.93	[1]	$ 0.97	[1]	$ 0.82	[1]	$ 0.83	[1]	$ 4.86	$ 3.55	$ 3.56

[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.