Quarterly Financial Information (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information [Abstract]
Revenue	$ 734,572	$ 779,474	$ 762,151	$ 696,245	$ 721,048	$ 743,586	$ 702,987	$ 620,276	$ 2,972,442	$ 2,787,897	$ 2,337,648
Operating income	113,922	139,854	140,899	103,565	112,298	126,262	122,695	80,052	498,240	441,307	338,438
Net income	$ 72,224	$ 84,368	$ 85,574	$ 62,524	$ 69,869	$ 77,909	$ 73,849	$ 45,887	$ 304,690	$ 267,514	$ 206,113
Basic and diluted	$ 0.85	$ 0.99	$ 1.00	$ 0.73	$ 0.81	$ 0.90	$ 0.86	$ 0.53	$ 3.57	$ 3.10