Discontinued Operations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Inventory assets	$ 161	$ 161	$ 153
Other current assets	81	81	81
Other current liabilities	253	253	272
Discontinued Operations [Member]
Derivative [Line Items]
Derivatives	0	0
Discontinued Operations [Member]
Derivative [Line Items]
Expected gain upon closing of contract	8	5
Expected gain upon closing of contract, after tax	5	3
Unrealized derivative losses after tax		(2)
Pre-tax unrealized derivative losses		(4)
Net accounts receivable	12	12	33
Inventory assets	2	2	3
Gross derivative assets	0	0	1
Other current assets	0	0	1
Accrued liabilities	9	9	20
Other current liabilities	3	3	1
Derivative liabilities	0	0	21
Outstanding commodity forward contracts or derivative positions	0	0
Discontinued Operations [Member] | Discontinued Operations [Member]
Derivative [Line Items]
Pre-tax loss associated with the terminations of the retail electric customer supply and wholesale purchase obligations	3
After-tax loss associated with the terminations of the retail electric customer supply and wholesale purchase obligations	$ 2