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REVENUE - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Trade and barter revenues $ 706,182 $ 780,170
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-04-01    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Revenue, remaining performance obligation $ 260,000  
Revenue, remaining performance obligation, period 5 years  
Advertising Trade and Barter Transactions    
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]    
Trade and barter revenues $ 2,200 $ 5,000