Quarterly report pursuant to Section 13 or 15(d)

Common Stock, Share-based Awards and Warrants - Narrative (Details)

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Common Stock, Share-based Awards and Warrants - Narrative (Details) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended
Jun. 25, 2018
Jul. 27, 2017
Jul. 27, 2016
Mar. 31, 2017
Aug. 31, 2016
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 21, 2017
Dec. 31, 2016
Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Stock repurchase program, authorized amount                         $ 50,000,000.0
Stock repurchase program, remaining authorized amount           $ 44,800,000   $ 44,800,000          
Income recorded as a result of remeasurement           $ (655,000) $ (445,000) $ (91,000) $ 2,475,000        
Equity Incentive Plan, 2013                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Number of shares authorized (in shares)                       11,000,000  
Equity Incentive Plan, 2013 | Employee Stock Options                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Shares available for grant                     2,097,846    
Equity Incentive Plan, 2017                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Number of shares authorized (in shares)                     6,500,000    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized 2,000,000                        
Public SPAC Warrants                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Warrants outstanding           0   0   6,173,000      
Common Stock | UMG Recordings                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Stock issued during period, litigation (in shares)         1,800,000                
Common Stock | Emerging Markets Communications LLC                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Stock issued during period, acquisitions (in shares)   5,000,000 5,500,000                    
Price per share of stock issued during period, acquisitions   $ 8.40                      
Public Warrants | Quotes prices in active markets (Level 1) | Public SPAC Warrants                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Number of warrants (in shares)             6,173,228   6,173,228        
Price Per Share Scenario Two | Common Stock | UMG Recordings                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Stock issued during period, litigation (in shares)       400,000 400,000.0                
Share price of common stock (in usd per share)         $ 12   $ 12.00   $ 12.00     $ 12.00  
Price Per Share Scenario One | Common Stock | UMG Recordings                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Stock issued during period, litigation (in shares)       500,000 500,000.0                
Share price of common stock (in usd per share)         $ 10   $ 10.00   $ 10.00     $ 10  
Liability Warrants                          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                          
Change in value             $ 200,000 $ 100,000 $ (200,000)