Fair Value Measurements (Summary Of Asset Impairment And Decommission Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Asset impairment	$ 3,020	$ 7,464	$ 6,033	$ 7,464
Total asset impairment and decommission costs	8,140	14,691	16,491	20,874
Former Corporate Headquarters Building [Member]
Write-offs relating to long-lived assets and intangibles		2,345		2,345
Not Identified by Segment [Member]
Write-offs relating to long-lived assets and intangibles	1,445	1,217	2,810	3,203
Total asset impairment and decommission costs		2,345		2,345
Decommissioned Towers [Member]
Write-offs relating to long-lived assets and intangibles	$ 3,675	$ 3,665	$ 7,648	$ 7,862