Schedule III - Schedule of Real Estate and Accumulated Depreciation (Reconciliation of Carrying Amount of Real Estate Investments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule III - Schedule of Real Estate and Accumulated Depreciation [Abstract]
Gross amount at beginning	$ 5,079,660	$ 4,839,874	$ 4,577,296
Acquisitions	112,979	72,456	203,441
Construction and related costs on new builds	70,361	58,143	87,088
Augmentation and tower upgrades	43,288	37,861	52,146
Land buyouts and other assets	41,657	44,574	47,148
Tower maintenance	29,391	28,257	27,123
Other		45,829
Total additions	297,676	287,120	416,946
Cost of real estate sold or disposed	(1,027)	(12,842)	(26,506)
Impairment	(34,102)	(34,491)	(34,373)
Other	(1,350)		(93,489)
Total deductions	(36,479)	(47,334)	(154,368)
Balance at end	$ 5,340,858	$ 5,079,660	$ 4,839,874