Fair Value Measurements (Summary Of Asset Impairment And Decommission Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Asset impairment	$ 4,128	$ 6,673	$ 10,162	$ 14,138
Total asset impairment and decommission costs	9,417	2,305	25,908	23,180
Former Corporate Headquarters Building [Member]
Write-offs relating to long-lived assets and intangibles				2,346
Not Identified by Segment [Member]
Write-offs relating to long-lived assets and intangibles	793	1,010	3,603	4,212
Total asset impairment and decommission costs				2,345
Decommissioned Towers [Member]
Write-offs relating to long-lived assets and intangibles	$ 4,496	3,587	$ 12,143	11,449
Tower Assets [Member]
Gain on sale of fiber assets		$ (8,965)		$ (8,965)