Revenue from Contracts with Customers - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of consolidated revenue		61.00%
Management fee, percentage		8.00%
Service fee retained by Sprint		8.60%
Management fee on prepaid wireless revenues and costs		6.00%
Wireless contract asset balance	$ 61.8	$ 61.8		$ 51.1
Payments to increase wireless contract asset balance	16.4	44.8
Amortization to reduce revenue	11.9	$ 34.1
Revenue recorded from equipment financing transactions			$ 63.8
Estimated customer life (in months)		42 months
Cost to fulfill contract	10.0	$ 10.0
Amortization of capitalized costs to fulfill contracts	4.1	$ 4.1
Cable
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Installation revenue, recognition period		10 months
Commissions and other costs, recognition period (in months)		44 months
Wireline
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Installation revenue, recognition period		11 months
Commissions and other costs, recognition period (in months)		72 months
Enterprise
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Commissions and other costs, recognition period (in months)		46 months
Prepaid expenses and other
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost to fulfill contract	4.6	$ 4.6
Deferred charges and other assets, net
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost to fulfill contract	$ 5.4	$ 5.4
Cost of Goods and Services Sold
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payments, cost of equipment and commissions			63.5
Selling, General and Administrative Expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payments, cost of equipment and commissions			$ 16.9
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payment, cost of equipment and commissions, benefit period for recognition		21 months
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payment, cost of equipment and commissions, benefit period for recognition		53 months