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Investments - Contractual Maturities (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Amortized Cost    
Due in one year or less $ 1,722  
Due after one year through five years 972  
Due after five years through ten years 6  
Amortized Cost 2,700 $ 1,761
Estimated Fair Value    
Due in one year or less 1,721  
Due after one year through five years 966  
Due after five years through ten years 6  
Estimated Fair Value $ 2,693