Debt and Financing Costs - Summary of Carrying Amounts and Estimated Fair Values (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Commercial paper and committed bank facilities, Carrying Amount	$ 0	$ 1,570,000,000
Total Debt, Carrying Amount	8,777,000,000	11,245,000,000
Total Debt, Fair Value	8,829,000,000	11,514,000,000
Commercial Paper and Committed Bank Facilities [Member]
Debt Instrument [Line Items]
Total Debt, Fair Value		1,570,000,000
Notes and Debentures [Member]
Debt Instrument [Line Items]
Notes and debentures, Carrying Amount	8,777,000,000	9,675,000,000
Total Debt, Fair Value	$ 8,829,000,000	$ 9,944,000,000