BUSINESS COMBINATIONS - Narrative (Details) - USD ($)										3 Months Ended		12 Months Ended
	Oct. 16, 2019	Jul. 19, 2019	May 09, 2019	Sep. 27, 2018	Aug. 02, 2018	Jul. 18, 2018	Apr. 30, 2018	Nov. 17, 2017	Jan. 06, 2017	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jul. 31, 2017
Business Acquisition [Line Items]
Payments to acquire business												$ 40,136,000	$ 71,434,000	$ 24,000,000
Adjustment to Acquired Goodwill, Deferred Tax Liability													3,300,000
Costs to exit duplicative contracts												0	229,000	500,000
Adjustments to Acquired Goodwill TIA													2,300,000
Adjustment to Acquired Goodwill Current Assets													1,300,000
Adjustments to Acquired Goodwill, Change in Purchase Price Allocation													16,400,000
Integration costs												$ 4,297,000	25,372,000	$ 0
Adjustments to Acquired Goodwill, Prepaid Assets													$ 500,000
Cadence 13
Business Acquisition [Line Items]
Consideration transferred	$ 24,300,000
Recognized gain on previously held equity interest	$ 5,300,000
Percentage of equity interest prior to acquisition															45.00%
Pineapple Street Media
Business Acquisition [Line Items]
Consideration transferred		$ 14,000,000.0
2019 Cumulus Exchange
Business Acquisition [Line Items]
Gain (loss) on disposition of business			$ 1,800,000
2019 Cumulus Exchange | Measurement Input, Discount Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input			9.00%
2019 Cumulus Exchange | Measurement Input, Long-term Revenue Growth Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input			1.00%
Jerry Lee
Business Acquisition [Line Items]
Payments to acquire business				$ 57,500,000
Jerry Lee | Measurement Input, Discount Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input				9.00%
Jerry Lee | Measurement Input, Long-term Revenue Growth Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input				1.00%
Beasley - WXTU
Business Acquisition [Line Items]
Gain (loss) on disposition of business										$ 4,400,000
Proceeds from divestiture of businesses						$ 38,000,000.0
Emmis
Business Acquisition [Line Items]
Payments to acquire business							$ 15,000,000.0
Emmis | Measurement Input, Discount Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input							9.00%
Emmis | Measurement Input, Long-term Revenue Growth Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input							1.00%
CBS Radio
Business Acquisition [Line Items]
Consideration transferred								$ 2,560,000,000
Assumed debt								1,390,000,000
Equity consideration transferred								$ 1,170,000,000
Issuance of Class A common stock in the Merge (in shares)														101,407,494
Beasley Transaction | Measurement Input, Discount Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input								9.00%
Beasley Transaction | Measurement Input, Long-term Revenue Growth Rate
Business Acquisition [Line Items]
Business combination assets acquired measurement input								1.00%
Bonneville Transaction
Business Acquisition [Line Items]
Gain (loss) on disposition of business					$ 400,000						$ (200,000)
Proceeds from divestiture of businesses					$ 141,000,000.0
2017 Charlotte Acquisition
Business Acquisition [Line Items]
Consideration transferred									$ 24,000,000
EMF Sale
Business Acquisition [Line Items]
Gain (loss) on disposition of business														$ 2,500,000
Proceeds from divestiture of businesses														57,800,000
Assets Acquired | Beasley Transaction
Business Acquisition [Line Items]
Cash Acquired in Excess of Payments to Acquire Business														$ 12,000,000.0