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Investments - Contractual Maturities of Investments (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Amortized Cost    
Due in one year or less $ 1,301  
Due after one year through five years 1,194  
Due after five years through ten years 23  
Amortized Cost 2,518 $ 1,761
Estimated Fair Value    
Due in one year or less 1,301  
Due after one year through five years 1,193  
Due after five years through ten years 23  
Total, Estimated Fair Value $ 2,517