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Fair Value (Details 1) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance $ 44 $ 59
Transfers into Level 3 1 4
Transfer out of Level 3 0 (24)
Total gains: [Abstract]    
Included in earnings 60 87
Recorded in regulatory assets/liabilities 15 2
Purchases issuances and settlements [Abstract]    
Purchases 2 3
Settlements (135) (87)
Ending Balance (13) 44
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 58 75
Natural Gas [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 6 1
Transfers into Level 3 1 0
Transfer out of Level 3 0 1
Total gains: [Abstract]    
Included in earnings (41) 7
Recorded in regulatory assets/liabilities 0 0
Purchases issuances and settlements [Abstract]    
Purchases 0 0
Settlements (4) (3)
Ending Balance (38) 6
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 (33) 8
Electricity [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 32 54
Transfers into Level 3 0 4
Transfer out of Level 3 0 (25)
Total gains: [Abstract]    
Included in earnings 101 77
Recorded in regulatory assets/liabilities 0 0
Purchases issuances and settlements [Abstract]    
Purchases 2 3
Settlements (112) (81)
Ending Balance 23 32
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 91 65
Other [Member]
   
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis    
Beginning Balance 6 4
Transfers into Level 3 0 0
Transfer out of Level 3 0 0
Total gains: [Abstract]    
Included in earnings 0 3
Recorded in regulatory assets/liabilities 15 2
Purchases issuances and settlements [Abstract]    
Purchases 0 0
Settlements (19) (3)
Ending Balance 2 6
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 $ 0 $ 2