Property, Plant and Equipment (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) Years	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Land and rights-of-way			$ 4.3	$ 4.3
Buildings and leasehold improvements			179.1	173.7
Network equipment			3,339.4	3,165.7
Office software, furniture, fixtures and vehicles			162.5	150.6
Construction in process			14.7	17.0
Gross value			3,700.0	3,511.3
Accumulated depreciation			(2,571.0)	(2,425.8)
Property, plant and equipment, net			1,129.0	1,085.5
Depreciation			$ 190.4	$ 182.0	$ 141.3
Depreciation associated with cost of providing services			84.00%	85.00%	79.00%
Impairment of assets, excluding goodwill			$ 0.0	$ 0.0	$ 0.0
Capital Lease and Other Financing Arrangements Assets, Gross			$ 112.0	96.8
Capital Leases, Net of Depreciation, Retained After Sale of Business, Affiliate and Productive Assets		$ 57.7
Land [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			20 years
Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Minimum in Practice | Years			7
Property, Plant and Equipment, Useful Life, Maximum in Practice | Years			22
copper assets [Member]
Property, Plant and Equipment [Line Items]
property, plant, and equipment, copper asset useful life, prior to remeasurement					15
property, plant, and equipment, copper assets useful life, after remeasurement					7
Entertainment and Communications [Member]
Property, Plant and Equipment [Line Items]
Impairment of assets, excluding goodwill	$ 0.0	$ 0.0		$ 0.0
Minimum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			3 years
Minimum [Member] | Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			2 years
Minimum [Member] | Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			2 years
Maximum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			40 years
Maximum [Member] | Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			50 years
Maximum [Member] | Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life			14 years