Annual report pursuant to Section 13 and 15(d)

Acquisitions and Recent Transactions - 2017 Acquisitions - Broadcast - Additional Information (Detail)

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Acquisitions and Recent Transactions - 2017 Acquisitions - Broadcast - Additional Information (Detail) - USD ($)
 12 Months Ended
Nov. 22, 2017
Sep. 15, 2017
Jul. 24, 2017
Jun. 28, 2017
Mar. 14, 2017
Mar. 01, 2017
Jan. 16, 2017
Jan. 06, 2017
Dec. 31, 2018
Dec. 31, 2017
Jan. 03, 2017
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business combination, consideration transferred                 $ 11,180,000 $ 4,043,000  
Goodwill                 $ 26,789,000 $ 26,424,000 $ 25,613,000
WWRC-AM Formerly WSPZ-AM in Bethesda, Maryland Business Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Nov. 22, 2017  
Business combination, consideration transferred                   $ 620,000  
Real Property of Radio Station WSPZ-AM in Bethesda, Maryland Business Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Sep. 15, 2017  
Business combination, consideration transferred                   $ 1,500,000  
FM Translator Construction Permit, Eaglemount, Washington Asset Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Jul. 24, 2017  
Business combination, consideration transferred                   $ 40,000  
FM Translator Construction Permit, Festus, Missouri Asset Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Jun. 28, 2017  
Business combination, consideration transferred                   $ 40,000  
FM Translator Construction Permit, Quartz Site, Arizona Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Mar. 14, 2017  
Business combination, consideration transferred                   $ 20,000  
FM Translator Construction Permit, Roseburg, Oregon Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Mar. 01, 2017  
Business combination, consideration transferred                   $ 45,000  
FM Translator, Astoria, Oregon Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Jan. 16, 2017  
Business combination, consideration transferred                   $ 33,000  
FM Translator Construction Permit, Mohave Valley, Arizona Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition                   Jan. 01, 2017  
Business combination, consideration transferred                   $ 20,000  
2017 Acquisitions Broadcast [Member] | WWRC-AM Formerly WSPZ-AM in Bethesda, Maryland Business Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition Nov. 22, 2017                    
Business combination, consideration transferred $ 600,000                    
2017 Acquisitions Broadcast [Member] | Real Property of Radio Station WSPZ-AM in Bethesda, Maryland Business Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition   Sep. 15, 2017                  
Business combination, consideration transferred   $ 1,500,000                  
Goodwill   $ 13,000                  
2017 Acquisitions Broadcast [Member] | FM Translator Construction Permit, Eaglemount, Washington Asset Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition     Jul. 24, 2017                
Business combination, consideration transferred     $ 40,000                
2017 Acquisitions Broadcast [Member] | FM Translator Construction Permit, Festus, Missouri Asset Acquisition [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition       Jun. 28, 2017              
Business combination, consideration transferred       $ 40,000              
2017 Acquisitions Broadcast [Member] | FM Translator Construction Permit, Quartz Site, Arizona Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition         Mar. 14, 2017            
Business combination, consideration transferred         $ 20,000            
2017 Acquisitions Broadcast [Member] | FM Translator Construction Permit, Roseburg, Oregon Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition           Mar. 01, 2017          
Business combination, consideration transferred           $ 45,000          
2017 Acquisitions Broadcast [Member] | FM Translator, Astoria, Oregon Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition             Jan. 16, 2017        
Business combination, consideration transferred             $ 33,000        
2017 Acquisitions Broadcast [Member] | FM Translator Construction Permit, Mohave Valley, Arizona Asset Purchase [Member]                      
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]                      
Business acquisition, Effective Date of Acquisition               Jan. 06, 2017      
Business combination, consideration transferred               $ 20,000