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Derivative Instruments and Hedging Activities (Notional Volumes) (Details) (Economic Hedges) | Jun. 30, 2012 GWh | Dec. 31, 2011 GWh
--- | --- | ---
Electricity options, swaps and forwards |  |
Derivative [Line Items] |  |
Nonmonetary notional amount of derivative | 26,597 | 30,881
Natural gas options, swaps and forwards |  |
Derivative [Line Items] |  |
Nonmonetary notional amount of derivative | 213 | 300
Congestion revenue rights |  |
Derivative [Line Items] |  |
Nonmonetary notional amount of derivative | 136,863 | 166,163
Tolling arrangements |  |
Derivative [Line Items] |  |
Nonmonetary notional amount of derivative | 102,819 | 104,154

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration