Quarterly report pursuant to Section 13 or 15(d)

Related Party Transactions - Pandora (Details)

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Related Party Transactions - Pandora (Details) - USD ($)
3 Months Ended 6 Months Ended 9 Months Ended
Sep. 23, 2018
Sep. 22, 2017
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2019
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Related Party Transaction [Line Items]                
Investment in convertible preferred stock           $ 7,720,000 $ 612,205,000  
Preferred stock, par value (in dollars per share)     $ 0.001     $ 0.001    
Agreement to Acquire Pandora, Inc.                
Common stock, par value (in dollars per share) $ 0.001   $ 0.001     $ 0.001   $ 0.001
Pandora | Termination Fee                
Agreement to Acquire Pandora, Inc.                
Agreement termination fee receivable $ 105,000,000              
Pandora | Termination Fee, Contract Termination On Or Before November 22, 2018                
Agreement to Acquire Pandora, Inc.                
Agreement termination fee receivable $ 52,500,000              
Pandora | Forecast                
Agreement to Acquire Pandora, Inc.                
Shares issuable per acquiree share (in shares)         1.44      
Pandora | Performance-based Share Awards | Maximum                
Agreement to Acquire Pandora, Inc.                
Consideration transferred, per share $ 20.00              
Pandora                
Related Party Transaction [Line Items]                
Fair value of investment     $ 554,352,000     $ 554,352,000   $ 480,472,000
Pandora | Level 2                
Related Party Transaction [Line Items]                
Fair value of investment     $ 554,352,000     $ 554,352,000   $ 480,472,000
Pandora                
Agreement to Acquire Pandora, Inc.                
Common stock, par value (in dollars per share) $ 0.0001              
Pandora | Series A Preferred Stock                
Related Party Transaction [Line Items]                
Number of shares issued (in shares)   480,000            
Investee | Pandora                
Related Party Transaction [Line Items]                
Investment in convertible preferred stock   $ 480,000,000            
Investment ownership percentage     18.00%     18.00%    
Ownership percentage on an as-converted basis     15.00%     15.00%    
Accrued dividends     $ 33,270,000     $ 33,270,000    
Unrealized gain (loss) on investment     $ (43,569,000) $ 72,245,000   $ 73,880,000 $ 72,245,000  
Investee | Pandora | Series A Preferred Stock                
Related Party Transaction [Line Items]                
Preferred stock, par value (in dollars per share)   $ 0.0001            
Price per share (in dollars per share)     $ 10.50     $ 10.50    
Common shares issued upon conversion (in shares)     95.2381     95.2381    
Preferred stock dividend rate, percentage           6.00%    
Liquidation preference     $ 513,270,000     $ 513,270,000