Other Material Contingencies - Schedule of Leveraged Lease Transactions Effect on Consolidated Income Statement (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Leveraged Lease [Line Items]
Total increase/(decrease) in net income	$ 26	$ (95)
Increase/(decrease) to non-utility operating revenues [Member]
Leveraged Lease [Line Items]
Total increase/(decrease) in net income	44	(27)
(Increase)/decrease to other interest expense [Member]
Leveraged Lease [Line Items]
Total increase/(decrease) in net income		(131)
Income tax benefit/(expense) [Member]
Leveraged Lease [Line Items]
Total increase/(decrease) in net income	$ (18)	$ 63