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FCC Licenses - Discounted Cash Flow Analyses (Detail) - CBS Radio Stations Inc Entercom Boston LLC and The Entercom Divestiture Trust [Member]
 3 Months Ended 12 Months Ended
Sep. 30, 2020
Mar. 31, 2020
Dec. 31, 2020
Measurement Input, Discount Rate [Member]      
Fair Value Inputs Asset Quantitative Information [Line Items]      
Fair value assumptions inputs rate 9.00% 9.50% 9.00%
Minimum [Member]      
Fair Value Inputs Asset Quantitative Information [Line Items]      
Market revenue shares at maturity 0.60% 0.60% 0.60%
Operating income margins at maturity 19.50% 26.50% 27.50%
Minimum [Member] | Measurement Input, Long-term Revenue Growth Rate [Member]      
Fair Value Inputs Asset Quantitative Information [Line Items]      
Fair value assumptions inputs rate 0.50% (14.10%) 0.50%
Maximum [Member]      
Fair Value Inputs Asset Quantitative Information [Line Items]      
Market revenue shares at maturity 44.40% 39.00% 1.10%
Operating income margins at maturity 33.30% 35.40% 29.80%
Maximum [Member] | Measurement Input, Long-term Revenue Growth Rate [Member]      
Fair Value Inputs Asset Quantitative Information [Line Items]      
Fair value assumptions inputs rate 18.80% 7.90% 1.40%