Other Material Contingencies - Schedule of Leveraged Lease Transactions Effect on Consolidated Income Statement (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Leveraged Lease [Line Items]
Total increase/(decrease) in net income	$ 29	$ (121)
Increase/(decrease) to non-utility operating revenues [Member]
Leveraged Lease [Line Items]
Total increase/(decrease) in net income	51	(70)
(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	$ (22)	$ 80