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Marketable Investment Securities - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Gain (Loss) on Securities [Line Items]          
Available-for-sale securities with contractual maturities of one year of less $ 979   $ 979    
Available-for-sale securities with contractual maturity of greater than one year 184   184    
Gain on investments 0 $ 23 4 $ 23  
Proceeds from sale of available-for-sale securities 0   436    
Gain (Loss) on Investments          
Gain (Loss) on Securities [Line Items]          
Gain (loss) on equity securities 13 $ 42 43 $ 5  
Corporate bonds          
Gain (Loss) on Securities [Line Items]          
Fair value of investments $ 7   $ 7   $ 48