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 9 Months Ended 12 Months Ended
Jul. 27, 2017
Jul. 27, 2016
Mar. 31, 2017
Aug. 31, 2016
Aug. 31, 2015
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2014
Jan. 18, 2018
Dec. 21, 2017
Jun. 30, 2017
Dec. 31, 2016
Mar. 31, 2016
Jan. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Stock repurchase program, authorized amount                             $ 50,000,000  
Stock repurchase program, remaining authorized amount           $ 44,800,000   $ 44,800,000                
Income recorded as a result of remeasurement           196,000 $ 1,191,000 2,672,000 $ 17,982,000              
Number of warrants authorized for repurchase                   $ 25,000,000.0            
Number of additional warrants authorized for repurchase         $ 20,000,000.0                      
Amount of available for warrants               16,700,000                
Equity Incentive Plan, 2013                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of shares authorized (in shares)                           11,000,000    
Public SPAC Warrants                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Warrants outstanding                         6,173,000      
Income recorded as a result of remeasurement           $ 100,000 $ 1,100,000 $ 300,000 $ 17,800,000              
Term of warrant               5 years                
Closing stock price for 20 or 30 days to trigger call of warrants (usd per share)               $ 17.50                
Class of warrant or right, threshold days               20 days                
Number of consecutive days               30 days                
Redemption by issuer, price (usd per share)           $ 0.01   $ 0.01                
Estimated proceeds if warrants called               $ 71,000,000                
Emerging Markets Communications LLC                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Deferred compensation to be paid   $ 25,000,000                            
Common Stock                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Stock issued during period, acquisitions (in shares)               5,080,000                
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                            
Stock issued during period, acquisitions (in usd per share) $ 8.40                              
Subsequent Event | Equity Incentive Plan, 2013                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Vested and expected to vest (in shares)                     7,070,298          
Subsequent Event | Equity Incentive Plan, 2013 | Employee Stock Options                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of shares available for grant                       2,097,846        
Subsequent Event | 2017 Equity Incentive Plan                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Number of shares authorized (in shares)                       6,500,000        
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)     $ 12.00 $ 12   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)     $ 10.00 $ 10   10.00   10.00                
Common Stock Warrants | Row 44 Warrants                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Warrants outstanding                               21,062,500
Number of shares issuable upon exercise of warrants                               721,897
Sponsor Warrants | Public SPAC Warrants                                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                
Exercise price per warrant           $ 11.50   $ 11.50                
Quotes Prices in Active Markets (Level 1) | Public Warrants | 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         6,173,228