Quarterly Financial Information (Summary Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Revenue	$ 745,738	$ 782,611	$ 755,435	$ 707,733	$ 734,572	$ 779,474	$ 762,151	$ 696,245	$ 2,991,517	$ 2,972,442	$ 2,787,897
Operating income	113,447	137,404	133,436	99,548	113,922	139,854	140,899	103,565	483,835	498,240	441,307
Net income	$ 68,511	$ 85,581	$ 81,388	$ 60,285	$ 72,224	$ 84,368	$ 85,574	$ 62,524	$ 295,765	$ 304,690	$ 267,514
Earnings Per Share, Basic	$ 0.83	$ 1.03	$ 0.98	$ 0.72	$ 0.85	$ 0.99	$ 1.00	$ 0.73	$ 3.56	$ 3.57	$ 3.10
Earnings Per Share, Diluted	$ 0.83	$ 1.03	$ 0.98	$ 0.72	$ 0.85	$ 0.99	$ 1.00	$ 0.73	$ 3.56	$ 3.57	$ 3.10