Quarterly Financial Information (Unaudited) (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						6 Months Ended			9 Months Ended			12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015		Mar. 31, 2015		Jun. 30, 2016	Jun. 30, 2015		Sep. 30, 2016	Sep. 30, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 1,971,333	$ 1,807,211	$ 1,811,056		$ 1,744,679		$ 1,855,914	$ 1,789,765		$ 2,019,727	$ 1,860,957		$ 7,654,184	$ 7,206,457	$ 7,128,928
Net income	$ 152,790	$ 119,511	$ 109,745		$ 118,307		$ 108,967	$ 123,035		$ 220,650	$ 152,607		$ 601,916	$ 503,694	$ 648,884
Net income per share
Basic (dollars per share)	$ 1.26	$ 0.97	$ 0.89	[1]	$ 0.94	[1]	$ 0.89	$ 0.99	[1]	$ 1.80	$ 1.24	[1]	$ 4.91	$ 4.06	$ 5.07
Diluted (dollars per share)	$ 1.20	$ 0.93	$ 0.85	[1]	$ 0.89	[1]	$ 0.85	$ 0.95	[1]	$ 1.72	$ 1.18	[1]	$ 4.68	$ 3.87	$ 4.86

[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.