Property and Equipment - Narrative (Details)		1 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) site market tower option	Sep. 30, 2015 USD ($) market	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense			$ 22,700,000	$ 20,000,000.0	$ 14,700,000
Marketing agent agreement, number of towers | tower	283
Disposal group, including discontinued operation, consideration		$ 21,600,000
Marketing agent agreement, fair value	$ 3,100,000	$ 3,100,000
Gain (loss) on disposition of assets			$ (397,000)	$ (282,000)	12,029,000
Marketing agent agreement, term			5 years
Other Noncurrent Assets
Property, Plant and Equipment [Line Items]
Long-term prepaid rent asset					7,311,000
Other Noncurrent Liabilities
Property, Plant and Equipment [Line Items]
Marketing agent agreement, fair value					3,111,000
Deferred gain on the sale of towers					7,311,000
Tower Sale
Property, Plant and Equipment [Line Items]
Number of radio towers sold | tower	43
Number of radio tower sites sold | site	41
Number of markets in which tower sites sold | market	28	28
Disposal group, period of continuing involvement after disposal	35 years
Disposal group, period of continuing involvement after disposal, initial term	20 years
Disposal group, period of continuing involvement after disposal, number of renewal options | option	3
Disposal group, period of continuing involvement after disposal, renewal term	5 years
Disposal group, intra-entity amounts, discontinued operation after disposal, annual rent expense	$ 41
Disposal group, intra-entity amounts, discontinued operation after disposal, annual rent expense per site	1
Fair value of assets in disposal group, property, plant and equipment	$ 25,800,000	$ 25,800,000
Gain (loss) on disposition of assets					$ 11,500,000