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Amortized Cost and Estimated Fair Value of Marketable Securities by Contractual Maturity (Detail) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Cost    
Due in one year or less $ 33  
Due after one year through three years 578  
Due after three years through five years 23  
Due after five years 75  
Marketable Securities, Debt Maturities, Amortized Cost, Total 709  
Equity securities 2  
Total Amortized Cost 711 $ 813
Estimated Fair Value    
Due in one year or less 33  
Due after one year through three years 578  
Due after three years through five years 23  
Due after five years 76  
Marketable Securities, Debt Maturities, Fair Value, Total 710  
Equity securities 2  
Total Estimated Fair Value $ 712 $ 810