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	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	$ 29	$ 7	$ (121)
Non-Utility Operating Revenues [Member]
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	51		(70)
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	$ (22)	$ (6)	$ 80