Acquisitions (Narrative) (Details) $ in Millions	1 Months Ended			12 Months Ended
	Jan. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Nov. 30, 2014 item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Business Acquisition [Line Items]
Number of towers acquired | item		941	1,641	1,425	531	893
Cash paid for acquisition						$ 525.8
Property and equipment						176.3
Intangible assets						351.0
Working capital adjustments						$ 1.5
Other Acquisitions [Member]
Business Acquisition [Line Items]
Number of towers acquired | item				1,425	531
Cash paid for acquisition					$ 214.7
Property and equipment		$ 114.7		$ 114.7	72.8
Intangible assets		345.3		345.3	144.4
Working capital adjustments		$ 3.8		$ 3.8	$ 2.5
Subsequent Event [Member]
Business Acquisition [Line Items]
Number of towers acquired | item	308
Cash paid for acquisition	$ 79.5