INTANGIBLE ASSETS AND GOODWILL (Details) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2018 number	Jun. 30, 2017 number	Jun. 30, 2018 USD ($) number	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Changes in Intangible assets [Line Items]
Goodwill				$ 871,454	$ 862,000
Market approach for step one goodwill analysis [Abstract]
Income model years used for the discounted cash flow approach | number			10
Broadcasting License Impairment Testing [Member]
Estimates and assumptions used for impairment test [Line Items]
Discount rates	9.00%	9.25%
Broadcasting License Impairment Testing [Member] | Maximum [Member]
Estimates and assumptions used for impairment test [Line Items]
Operating profit margin ranges of the markets of the Company	37.00%	40.00%
Long-term revenue growth rate ranges of the markets of the Company	1.00%	2.00%
Broadcasting License Impairment Testing [Member] | Minimum [Member]
Estimates and assumptions used for impairment test [Line Items]
Operating profit margin ranges of the markets of the Company	22.00%	19.00%
Long-term revenue growth rate ranges of the markets of the Company	0.50%	1.00%
Goodwill Impairment Testing [Member]
Estimates and assumptions used for impairment test [Line Items]
Discount rates	9.00%	9.25%
Goodwill Impairment Testing [Member] | Maximum [Member]
Estimates and assumptions used for impairment test [Line Items]
Long-term revenue growth rate ranges of the markets of the Company	1.00%	2.00%
Market approach for step one goodwill analysis [Abstract]
Estimates of market multiples | number	0	8
Goodwill Impairment Testing [Member] | Minimum [Member]
Estimates and assumptions used for impairment test [Line Items]
Long-term revenue growth rate ranges of the markets of the Company	1.00%	1.00%
Market approach for step one goodwill analysis [Abstract]
Estimates of market multiples | number	0	7.5
Radio Broadcasting Licences [Member]
Changes in Intangible assets [Line Items]
Beginning of period balance			$ 2,649,959	2,649,959	823,195
Dispositions				0	(13,500)
Consolidation of a VIE				0	(15,738)
Acquisition of radio stations				0	17,174
Acquisition of radio stations - CBS Radio Merger				0	1,880,400
Disposition of Licenses - EMF Sale				0	(54,661)
Acqusition of radio Stations - Beasley Transaction				0	35,944
Acquisition of radio stations - iHeartMedia Transaction				0	50,621
Disposition of radio stations - iHeartMedia Transaction				0	(7,462)
Assets held for sale				0	(66,014)
Acquisition of radio stations - Emmis Acquisition				12,785	0
Acquisition Of Radio Station - Jerry Lee Acquisition				27,346	0
Impairment loss				(702)	0
Disposition Of Radio Stations -WXTU Transaction				(25,607)	0
Ending period balance				2,663,781	2,649,959
Goodwill [Member]
Changes in Intangible assets [Line Items]
Goodwill before cumulative loss on impairment					988,056	$ 158,333
Accumulated loss on impairment					(126,056)	(125,615)
Goodwill				871,454	862,000	$ 32,718
Acquisition of radio stations				0	43
Acquisition of radio stations - CBS Radio Merger				0	820,961
Disposition of Licenses - EMF Sale				0	(266)
Acqusition of radio Stations - Beasley Transaction				0	289
Acquisition of radio stations - iHeartMedia Transaction				0	11,700
Disposition of radio stations - iHeartMedia Transaction				0	(14)
Assets held for sale				0	(2,990)
Acquisition of radio stations - Emmis Acquisition				332	0
Acquisition Of Radio Station - Jerry Lee Acquisition				24,396	0
Disposition Of Radio Stations -WXTU Transaction				(8,623)	0
Goodwill, Impairment Loss				0	(441)
Adjustments to Acquired Goodwill				$ (6,651)	$ 0