Debt and Financing Costs - Summary of Carrying Amounts and Estimated Fair Values (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	$ 9,094	$ 9,183
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	8,479	8,544
Notes and Debentures [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	9,094	9,183
Notes and Debentures [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	8,479	8,544
Commercial Paper [Member] | Commercial Paper and Committed Bank Facilities [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	0	0
Commercial Paper [Member] | Commercial Paper and Committed Bank Facilities [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt instrument	$ 0	$ 0