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Fair Value (Details 1) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance $ 59 $ 24
Transfers into Level 3 4 9
Transfer out of Level 3 (24) 35
Total gains: [Abstract]    
Included in earnings 87 69
Recorded in regulatory assets/liabilities 2  
Purchases issuances and settlements [Abstract]    
Purchases 3  
Settlements (87)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   6
Purchases, issuances and settlements   (87)
Ending Balance 44 59
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period 75 (9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net   47
Natural Gas [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 1 2
Transfers into Level 3 0  
Transfer out of Level 3 1  
Total gains: [Abstract]    
Included in earnings 7 4
Recorded in regulatory assets/liabilities 0  
Purchases issuances and settlements [Abstract]    
Purchases 0  
Settlements (3)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   0
Purchases, issuances and settlements   (8)
Ending Balance 6 1
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period 8 (4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net   3
Electricity [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 54 19
Transfers into Level 3 4  
Transfer out of Level 3 (25)  
Total gains: [Abstract]    
Included in earnings 77 64
Recorded in regulatory assets/liabilities 0  
Purchases issuances and settlements [Abstract]    
Purchases 3  
Settlements (81)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   0
Purchases, issuances and settlements   (73)
Ending Balance 32 54
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period 65 (6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net   44
Other [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 4 3
Transfers into Level 3 0  
Transfer out of Level 3 0  
Total gains: [Abstract]    
Included in earnings 3 1
Recorded in regulatory assets/liabilities 2  
Purchases issuances and settlements [Abstract]    
Purchases 0  
Settlements (3)  
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)   6
Purchases, issuances and settlements   (6)
Ending Balance 6 4
The amount of total gains (losses) included in net income attributed to the change in unrealized gains or losses related to assets still held at the end of the period 2 1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net   $ 0