Discontinued Operations - Fair Value of Derivative Instruments by Balance Sheet Location (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Total Derivative assets	$ 4
Liabilities associated with assets held for disposition (current liabilities)		(4)
Liabilities associated with assets held for disposition (non-current liabilities)	(14)	(11)
Total Derivative liabilities	(14)
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Total Derivative assets
Liabilities associated with assets held for disposition (current liabilities)
Liabilities associated with assets held for disposition (non-current liabilities)
Total Derivative liabilities
Net Derivative (liability) asset
Other Derivative Instruments [Member]
Derivative [Line Items]
Total Derivative assets	4	8
Liabilities associated with assets held for disposition (current liabilities)	(1)	(4)
Liabilities associated with assets held for disposition (non-current liabilities)	(14)	(11)
Total Derivative liabilities	(15)	(15)
Net Derivative (liability) asset	(11)	(7)
Gross Derivative Instruments [Member]
Derivative [Line Items]
Total Derivative assets	4	8
Liabilities associated with assets held for disposition (current liabilities)	(1)	(4)
Liabilities associated with assets held for disposition (non-current liabilities)	(14)	(11)
Total Derivative liabilities	(15)	(15)
Net Derivative (liability) asset	(11)	(7)
Effects of Cash Collateral and Netting [Member]
Derivative [Line Items]
Total Derivative assets
Liabilities associated with assets held for disposition (current liabilities)	1
Liabilities associated with assets held for disposition (non-current liabilities)
Total Derivative liabilities	1
Net Derivative (liability) asset	1
Net Derivative Instruments [Member]
Derivative [Line Items]
Total Derivative assets	4	8
Liabilities associated with assets held for disposition (current liabilities)		(4)
Liabilities associated with assets held for disposition (non-current liabilities)	(14)	(11)
Total Derivative liabilities	(14)	(15)
Net Derivative (liability) asset	(10)	(7)
Discontinued Operations [Member] | Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Assets held for disposition (current assets)
Total Derivative assets
Liabilities associated with assets held for disposition (current liabilities)		(10)
Liabilities associated with assets held for disposition (non-current liabilities)		(1)
Total Derivative liabilities		(11)
Net Derivative (liability) asset		(11)
Discontinued Operations [Member] | Other Derivative Instruments [Member]
Derivative [Line Items]
Assets held for disposition (current assets)		1
Total Derivative assets		1
Liabilities associated with assets held for disposition (current liabilities)		(9)
Liabilities associated with assets held for disposition (non-current liabilities)		(1)
Total Derivative liabilities		(10)
Net Derivative (liability) asset		(9)
Discontinued Operations [Member] | Gross Derivative Instruments [Member]
Derivative [Line Items]
Assets held for disposition (current assets)		1
Total Derivative assets		1
Liabilities associated with assets held for disposition (current liabilities)		(19)
Liabilities associated with assets held for disposition (non-current liabilities)		(2)
Total Derivative liabilities		(21)
Net Derivative (liability) asset		(20)
Discontinued Operations [Member] | Effects of Cash Collateral and Netting [Member]
Derivative [Line Items]
Assets held for disposition (current assets)
Total Derivative assets
Liabilities associated with assets held for disposition (current liabilities)		16
Liabilities associated with assets held for disposition (non-current liabilities)		2
Total Derivative liabilities		18
Net Derivative (liability) asset		18
Discontinued Operations [Member] | Net Derivative Instruments [Member]
Derivative [Line Items]
Assets held for disposition (current assets)		1
Total Derivative assets		1
Liabilities associated with assets held for disposition (current liabilities)		(3)
Liabilities associated with assets held for disposition (non-current liabilities)
Total Derivative liabilities		(3)
Net Derivative (liability) asset		$ (2)