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Investments - Contractual Maturities of Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Amortized Cost    
Due in one year or less $ 1,576  
Due after one year through five years 959  
Due after five years through ten years 1  
Amortized Cost 2,536 $ 2,700
Estimated Fair Value    
Due in one year or less 1,573  
Due after one year through five years 947  
Due after five years through ten years 1  
Estimated Fair Value $ 2,521