Concentration Of Credit Risk (Summary Of Significant Customers And Percentage Of Total Revenue For Specified Time Periods Derived From Such Customers) (Details) - Revenue [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AT&T [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	25.00%	25.70%	24.20%
T-Mobile [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	16.50%	17.00%	16.00%
Verizon Wireless [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	15.20%	15.20%	13.80%
Sprint [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	15.10%	16.10%	19.60%
Domestic Site Leasing [Member] | AT&T [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	32.70%	32.70%	31.90%
Domestic Site Leasing [Member] | T-Mobile [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	19.70%	19.60%	19.00%
Domestic Site Leasing [Member] | Verizon Wireless [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	19.00%	18.20%	16.30%
Domestic Site Leasing [Member] | Sprint [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	18.90%	19.80%	22.30%
International Site Leasing [Member] | Oi S.A. [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	42.20%	43.90%	48.80%
International Site Leasing [Member] | Telefonica [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	25.70%	26.40%	24.70%
International Site Leasing [Member] | Claro [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	10.00%	9.40%	8.00%
Site Development Revenue [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	6.00%
Site Development Revenue [Member] | T-Mobile [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	26.90%	28.40%	17.60%
Site Development Revenue [Member] | Verizon Wireless [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	12.80%	16.50%	14.80%
Site Development Revenue [Member] | Sprint [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	12.90%	11.70%	28.50%
Site Development Revenue [Member] | Nokia, Inc. [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	10.10%	7.10%	6.30%
Site Development Revenue [Member] | Ericsson, Inc. [Member]
Concentration Risk [Line Items]
Concentration risk percentage of revenue	7.40%	5.00%	15.30%