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Derivative Instruments and Hedging Activities (Notional Volumes) (Details) (Economic Hedges)	Sep. 30, 2012 GWh	Dec. 31, 2011 GWh
Electricity options, swaps and forwards
Derivative [Line Items]
Nonmonetary notional amount of derivative	18,304	30,881
Natural gas options, swaps and forwards
Derivative [Line Items]
Nonmonetary notional amount of derivative	142	300
Congestion revenue rights
Derivative [Line Items]
Nonmonetary notional amount of derivative	140,263	166,163
Tolling arrangements
Derivative [Line Items]
Nonmonetary notional amount of derivative	102,123	104,154

X

- 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

Name:	invest_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration