Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Assets and Liabilities at Fair Value (Details)

v3.19.1
Summary of Significant Accounting Policies - Assets and Liabilities at Fair Value (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt $ 7,403     $ 6,633
Investment 5 $ 0    
Fair Value, Inputs, Level 1        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt 0     0
Fair Value, Inputs, Level 2        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt 7,403     6,633
Fair Value, Inputs, Level 3        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt 0     0
Pandora        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Pandora investment 0     523
Pandora | Fair Value, Inputs, Level 1        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Pandora investment 0     0
Pandora | Fair Value, Inputs, Level 2        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Pandora investment 0     523
Pandora | Fair Value, Inputs, Level 3        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Pandora investment $ 0     $ 0
Investee | Pandora        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Investment     $ 480