Revenue Recognition - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of consolidated revenue	59.00%
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	$ 43,200,000		$ 42,800,000
Payments to increase wireless contract asset balance	13,800,000
Amortization to reduce revenue	$ 13,400,000
Revenue recorded from equipment financing transactions		$ 63,800,000
Estimated customer life (in months)	42 months
Change in accounting principle - adoption of accounting standard (Note 2)	$ 50,035,000
Cost to fulfill contract	9,700,000
Amortization of capitalized costs to fulfill contracts	1,300,000
Impairment loss recognized, capitalized cost to fulfill contracts	$ 0
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	$ 4,600,000
Deferred charges and other assets, net
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost to fulfill contract	$ 5,100,000
Cost of Goods and Services Sold
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payments, cost of equipment and commissions		63,500,000
Selling, General and Administrative Expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reimbursement payments, cost of equipment and commissions		$ 16,900,000