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Investments - Contractual Maturities (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Amortized Cost    
Due in one year or less $ 1,000  
Due after one year through five years 692  
Amortized Cost 1,692 $ 2,700
Estimated Fair Value    
Due in one year or less 997  
Due after one year through five years 684  
Estimated Fair Value $ 1,681