Leases - Schedule of Leveraged Lease Transactions Effect on Consolidated Income Statement (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Operating Leased Assets [Line Items]
Increase/(decrease) in net income	$ (1)	$ (95)
Non Utility Operating Revenue [Member]
Operating Leased Assets [Line Items]
Increase/(decrease) in net income		(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	$ (14)	$ 63