PROPERTY, PLANT AND EQUIPMENT, INTANGIBLE ASSETS AND GOODWILL - Acquisitions and Dispositions (Narrative) (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) station	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Goodwill		$ 4,051,082		$ 4,066,575	$ 4,128,887
Net loss on sale of joint venture			$ 12,100
Cumulative translation adjustment from sale			$ 6,300
Disposed of by sale | Americas outdoor, Indianapolis, Indiana market
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale	$ 43,100
Net gain on exchange of assets	28,900
Disposed of by sale | Broadcast communication tower sites and related assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain on exchange of assets		15,400
Acquisition Of Certain Assets In Atlanta, Georgia From Fairway Media Group, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Fair value of assets acquired	39,400
Amount of fixed assets acquired	9,900
Amount of intangible assets acquired	29,500
Goodwill	$ 2,300
Acquisition Of Radio Stations From Entercom Communications Corp.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of fixed assets acquired		8,100
Amount of intangible assets acquired		63,200
Goodwill		$ 2,400
Chattanooga, TN | Acquisition Of Radio Stations From Entercom Communications Corp. | Broadcast communication tower sites and related assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of radio stations exchanged | station		4
Richmond, VA | Acquisition Of Radio Stations From Entercom Communications Corp. | Broadcast communication tower sites and related assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of radio stations exchanged | station		6
Boston, MA | Acquisition Of Radio Stations From Entercom Communications Corp. | Broadcast communication tower sites and related assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of radio stations acquired | station		4
Seattle, WA | Acquisition Of Radio Stations From Entercom Communications Corp. | Broadcast communication tower sites and related assets
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of radio stations acquired | station		3