Fair Value is Compared to Carrying Value Excluding Impact of Interest Rate Derivatives (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	$ 463	$ 0
Long-term Debt, Level 1	9,727	8,849
Long-term Debt, Level 2	6,007	1,749
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	481	0
Long-term Debt, Level 1	10,299	9,709
Long-term Debt, Level 2	$ 5,801	$ 1,690