Segment Information (Summary Of Inter-Segment Eliminations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues, net (including revenues, net from related parties of $5,578, $6,168 and $15,873, respectively)	$ 772,846	$ 696,875	$ 761,385	$ 740,823	$ 726,934	$ 648,023	$ 710,545	$ 720,189	$ 2,971,929	$ 2,805,691	$ 2,755,654
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues, net (including revenues, net from related parties of $5,578, $6,168 and $15,873, respectively)									(39,702)	(19,106)	(15,382)
Intersegment Eliminations [Member] | National Networks [Member]
Segment Reporting Information [Line Items]
Revenues, net (including revenues, net from related parties of $5,578, $6,168 and $15,873, respectively)									(33,600)	(17,634)	(14,963)
Intersegment Eliminations [Member] | International And Other [Member]
Segment Reporting Information [Line Items]
Revenues, net (including revenues, net from related parties of $5,578, $6,168 and $15,873, respectively)									$ (6,102)	$ (1,472)	$ (419)