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Investments - Contractual Maturities of Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Amortized Cost    
Due in one year or less $ 498  
Due after one year through five years 875  
Due after five years through ten years 107  
Due after ten years 271  
Amortized Cost 1,751 $ 1,692
Estimated Fair Value    
Due in one year or less 498  
Due after one year through five years 878  
Due after five years through ten years 108  
Due after ten years 273  
Estimated Fair Value $ 1,757