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Investments - Contractual Maturities of Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Amortized Cost    
Due in one year or less $ 1,356  
Due after one year through five years 914  
Amortized Cost 2,270 $ 2,700
Estimated Fair Value    
Due in one year or less 1,354  
Due after one year through five years 902  
Estimated Fair Value $ 2,256