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Acquisition And Divestitures - Additional Information (Detail) (Three Hundred Fifty One Megawatt Tapoco Hydroelectric Project, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 acre mi ElectricStation	Dec. 31, 2011
Three Hundred Fifty One Megawatt Tapoco Hydroelectric Project
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	169	146
Allocation of goodwill	$ 94

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Length Of Transmission Line

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Number Of Hydroelectric Generating Stations And Dams

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Purchase price allocation adjustment for allocation not yet complete at the prior year end.

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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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.

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

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-Name Accounting Standards Codification

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