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Investments - Contractual Maturities of Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Amortized Cost    
Due in one year or less $ 1,154  
Due after one year through five years 944  
Due after five years through ten years 17  
Amortized Cost 2,115 $ 1,761
Estimated Fair Value    
Due in one year or less 1,153  
Due after one year through five years 943  
Due after five years through ten years 17  
Total, Estimated Fair Value $ 2,113