Summary Of Significant Accounting Policies (Schedule Of Asset Classes And Related Estimated Useful Lives) (Details)	12 Months Ended
	Dec. 31, 2017
Towers and Related Components [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	3 years
Towers and Related Components [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	15 years
Furniture, equipment and vehicles [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	2 years
Furniture, equipment and vehicles [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	7 years
Buildings and improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	10 years
Buildings and improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives	30 years