Quarterly Financial Information (Summary Of Quarterly Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Revenue	$ 527,345	$ 544,493	$ 541,505	$ 497,140	$ 485,132	$ 494,475	$ 480,255	$ 422,679	$ 2,110,483	$ 1,882,541	$ 1,480,998
Operating income	67,516	80,932	82,588	54,218	63,312	68,194	64,645	37,921	285,254	234,072	137,739
Net income	$ 39,481	$ 51,044	$ 47,832	$ 31,095	$ 39,883	$ 38,631	$ 39,383	$ 21,573	$ 169,452	$ 139,470	$ 75,651
Basic and diluted	$ 0.46	$ 0.59	$ 0.56	$ 0.36	$ 0.46	$ 0.45	$ 0.46	$ 0.26	$ 1.97	$ 1.63