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Carrying Values and Estimated Fair Values of Financial Instruments (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Carrying (Reported) Amount, Fair Value Disclosure    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 13,821 $ 13,268
Estimate of Fair Value, Fair Value Disclosure    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value 14,068 13,787
Estimate of Fair Value, Fair Value Disclosure | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value 10,377 10,115
Estimate of Fair Value, Fair Value Disclosure | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt fair value $ 3,691 $ 3,672