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Fair Value Measurements (Level 3 Reconciliation) (Details) (Price Risk Management Instruments [Member], Fair Value, Inputs, Level 3 [Member], USD $)
In Millions
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Price Risk Management Instruments [Member] | Fair Value, Inputs, Level 3 [Member]
    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Liability balance$ (280)$ (470)$ (399)$ (250)
Realized and unrealized gains (losses) included in cost of electricity or cost of natural gas4[1](14)[1]20[1](76)[1]
Realized and unrealized gains (losses) included in regulatory assets and liabilities(89)(223)(190)(558)
Purchases5349153141
Settlements3160135145
Liability balance$ (281)$ (598)$ (281)$ (598)
[1] Balancing account revenue is recorded for these amounts, therefore, there is no impact to net income.