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Marketable Investment Securities - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Gain (Loss) on Securities [Line Items]      
Available-for-sale securities with contractual maturities of one year of less $ 1,600,000,000    
Available-for-sale securities with contractual maturity of greater than one year 186,000,000    
Equity securities 64,887,000   $ 90,976,000
Proceeds from sale of available-for-sale securities 436,000,000 $ 0  
Corporate bonds      
Gain (Loss) on Securities [Line Items]      
Fair value of investments 53,000,000 0  
Gain on investments 4,000,000    
Gain (Loss) on Investments      
Gain (Loss) on Securities [Line Items]      
Gain (loss) on equity securities $ 30,000,000 $ 37,000,000