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Fair Value (Details 1) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis        
Beginning Balance $ 57 $ 65 $ 44 $ 59
Transfers into Level 3 0 (7) 29 (4)
Transfer out of Level 3 0 (42) (2) (24)
Total gains: [Abstract]        
Included in earnings (30) 29 15 39
Recorded in regulatory assets/liabilities 7 0 12 3
Purchases issuances and settlements [Abstract]        
Purchases 0 0 1 1
Settlements (48) (36) (113) (65)
Ending Balance (14) 9 (14) 9
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 (37) 11 4 24
Natural Gas [Member]
       
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis        
Beginning Balance 2 1 6 1
Transfers into Level 3 0 (1) 1 0
Transfer out of Level 3 0 0 (2) 1
Total gains: [Abstract]        
Included in earnings (43) 6 (38) 3
Recorded in regulatory assets/liabilities 0 0 0 0
Purchases issuances and settlements [Abstract]        
Purchases 0 0 0 0
Settlements (3) (1) (11) 0
Ending Balance (44) 5 (44) 5
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 (43) 6 (39) 5
Electricity [Member]
       
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis        
Beginning Balance 50 57 32 54
Transfers into Level 3 0 (6) 28 (4)
Transfer out of Level 3 0 (42) 0 (25)
Total gains: [Abstract]        
Included in earnings 12 23 53 34
Recorded in regulatory assets/liabilities 0 0 0 0
Purchases issuances and settlements [Abstract]        
Purchases 0 0 1 1
Settlements (38) (35) (90) (63)
Ending Balance 24 (3) 24 (3)
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 5 5 43 17
Other Credit Derivatives [Member]
       
Fair value reconciliation of Level 3 assets and liabilities measured at fair value on a recurring basis        
Beginning Balance 5 7 6 4
Transfers into Level 3 0 0 0 0
Transfer out of Level 3 0 0 0 0
Total gains: [Abstract]        
Included in earnings 1 0 0 2
Recorded in regulatory assets/liabilities 7 0 12 3
Purchases issuances and settlements [Abstract]        
Purchases 0 0 0 0
Settlements (7) 0 (12) (2)
Ending Balance 6 7 6 7
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 $ 1 $ 0 $ 0 $ 2