Acquisition and Divestitures - Additional Information (Detail) (Three Hundred Fifty One Megawatt Tapoco Hydroelectric Project [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012 acre mi ElectricStation	Dec. 31, 2011
Three Hundred Fifty One Megawatt Tapoco Hydroelectric Project [Member]
Business Acquisition [Line Items]
Sale of energy	$ 600
Number of hydroelectric generating stations and dams	4
Length of transmission line	86
Acres of land associated with divestitures	14,500
Net carrying value of the assets	177	146
Allocation of goodwill	$ (94)

The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3179-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 15, 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration