Fair Value Measurements (Summary Of Asset Impairment And Decommission Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset impairment	$ 15,389	$ 19,217	$ 10,287
Gain on sale of fiber assets		(8,919)
Other third party decommission costs	4,447	4,632	5,378
Total asset impairment and decommission costs	36,697	30,242	94,783
Former Corporate Headquarters Building [Member]
Write-offs relating to long-lived assets and intangibles		2,345	1,154
Fiber Assets [Member]
Write-offs relating to long-lived assets and intangibles			56,733
Decommissioned Towers [Member]
Write-offs relating to long-lived assets and intangibles	$ 16,861	$ 12,967	$ 21,231