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Amortized Cost and Estimated Fair Value of Marketable Securities by Contractual Maturity (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Cost    
Due in one year or less $ 129  
Due after one year through three years 434  
Due after three years through five years 22  
Due after five years 84  
Marketable Securities, Debt Maturities, Amortized Cost, Total 669  
Equity securities 2  
Total Amortized Cost 671 $ 736
Estimated Fair Value    
Due in one year or less 129  
Due after one year through three years 429  
Due after three years through five years 21  
Due after five years 84  
Marketable Securities, Debt Maturities, Fair Value, Total 663  
Equity securities 2  
Total Estimated Fair Value $ 665 $ 749