Other Material Contingencies - Schedule of Leveraged Lease Transactions Effect on Consolidated Income Statement (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Leveraged Lease [Line Items]
Total reduction in net income	$ 150
Reduction to non-utility operating revenues [Member]
Leveraged Lease [Line Items]
Total reduction in net income	121
Increase to other interest expense [Member]
Leveraged Lease [Line Items]
Total reduction in net income	131
Income tax expense [Member]
Leveraged Lease [Line Items]
Total reduction in net income	$ (102)