Fair Value Measurements (Summary of Asset Impairment and Decommission Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Asset impairment	$ 4,282	$ 4,526	$ 7,585	$ 10,382
Other (including third party decommission costs)	331	508	642	1,156
Total asset impairment and decommission costs	9,620	7,404	15,391	15,909
Decommissioned Towers [Member]
Write-off carrying value of decommissioned towers	$ 5,007	$ 2,370	$ 7,164	$ 4,371