Lease In/Lease Out Transactions - Schedule of Effect of LILO Transactions on Consolidated Income Statement (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	$ 26	$ 7	$ (95)
Non Utility Operating Revenue [Member]
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	44		(27)
Interest Expense [Member]
Operating Leased Assets [Line Items]
Increase/(decrease) in net income		13	(131)
Income Tax Benefit/(Expense) [Member]
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	$ (18)	$ (6)	$ 63