Derivative Instruments and Hedging Activities - Changes in Fair Values of Commodity Derivatives (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended	3 Months Ended
Sep. 30, 2014 Purchased Power Costs [Member]	Sep. 30, 2014 Purchased Power Costs [Member]	Sep. 30, 2013 Purchased Power Costs [Member]	Sep. 30, 2013 Non Utility Operating Revenue [Member]
Derivatives, Fair Value [Line Items]
Unrealized gain/(loss) on derivatives	$ 1	$ 14	$ 22	$ 6

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

Name:	us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration