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Derivatives and Risk Management Activities (Details 6) (USD $)
In Millions
 6 Months Ended 12 Months Ended 3 Months Ended 6 Months Ended 6 Months Ended
Jun. 30, 2011
Dec. 31, 2010
Jun. 30, 2011
Commodity
Level 1
Dec. 31, 2010
Commodity
Level 1
Jun. 30, 2011
Level 1
Dec. 31, 2010
Level 1
Jun. 30, 2011
Interest Rate Contracts
Level 2
Jun. 30, 2011
Level 2
Jun. 30, 2011
Commodity
Level 3
Dec. 31, 2010
Commodity
Level 3
Dec. 31, 2010
Interest Rate Contracts
Level 3
Dec. 31, 2010
Foreign exchange contracts
Level 3
Jun. 30, 2011
Level 3
Jun. 30, 2010
Level 3
Jun. 30, 2011
Level 3
Jun. 30, 2010
Level 3
Dec. 31, 2010
Level 3
Jun. 30, 2011
Commodity
Total
Dec. 31, 2010
Commodity
Total
Jun. 30, 2011
Interest Rate Contracts
Total
Dec. 31, 2010
Interest Rate Contracts
Total
Dec. 31, 2010
Foreign exchange contracts
Total
Jun. 30, 2011
Total
Dec. 31, 2010
Total
Jun. 30, 2011
Interest rate and commodity derivatives
Derivatives and Risk Management Activities                                                  
Broker receivable resulting from cash collateral deposited $ 33 $ 99                                              
Initial margin 65 56                                              
Variation margin return 32 43                                              
Recurring Fair Value Measures                                                  
Net asset/(liability) at fair value     12 (16) 12 (16) 7 7 10 (30) 15 1 10   10   (14) 22 (46) 7 15 1 29 (30)  
Roll forward of Level 3 Net Liability                                                  
Beginning Balance                         (5) (5) (14) (28)                  
Unrealized gains/(losses):                                                  
Included in earnings                         7 5 13 12                  
Included in other comprehensive income                         1 1 (1) 1                  
Settlements                         1 6 33 27                  
Derivatives entered into during the period                         6 1 (4) (4)                  
Transfers out of Level 3                             (17)                    
Ending Balance                         10 8 10 8                  
Change in unrealized gains/(losses) included in earnings relating to level 3 derivatives still held at the end of the periods                         13 10 11 9                  
Transfers from level 3 to Level 2                                                 $ 17