Derivatives and Other Financial Instruments - Gross Amounts of Recognized Derivative Assets and Liabilities and Gross Amounts Offset in Accompanying Consolidated Balance Sheet (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Offsetting [Abstract]
Margin on derivative posted with counterparties	$ 18	$ 9