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Debt and Financing Costs - Summary of Carrying Amounts and Estimated Fair Values (Detail) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value $ 8,561 $ 8,330
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying Amount 8,719 8,717
Notes and Debentures [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 8,561 8,330
Notes and Debentures [Member] | Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying Amount 8,719 8,717
Commercial Paper [Member] | Commercial Paper and Committed Bank Facilities [Member] | Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0 0
Commercial Paper [Member] | Commercial Paper and Committed Bank Facilities [Member] | Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Carrying Amount $ 0 $ 0