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Accounting for Derivative Instruments and Hedging Activities - Offsetting of Derivatives by Counterparty Master Agreement level and Collateral Received (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets $ 798 $ 1,248
Cash Collateral (Held)/Posted (37) (81)
Derivative liabilities 752 1,376
Cash collateral posted in support of energy risk management activities 171 150
Gross Amounts of Recognized Assets / Liabilities 46 (128)
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 62 0
Net Amount 108 (128)
Commodity contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 745 1,199
Derivative Instruments (578) (1,021)
Cash Collateral (Held)/Posted (11) (13)
Net Amount 156 165
Derivative liabilities 693 1,288
Derivative Instruments 578 1,021
Cash collateral posted in support of energy risk management activities 73 13
Net Amount (42) (254)
Gross Amounts of Recognized Assets / Liabilities 52 (89)
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 62 0
Net Amount 114 (89)
Interest rate contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 53 49
Derivative Instruments (3) (4)
Cash Collateral (Held)/Posted 0
Net Amount 50 45
Derivative liabilities 59 88
Derivative Instruments 3 4
Cash collateral posted in support of energy risk management activities 0 0
Net Amount (56) (84)
Gross Amounts of Recognized Assets / Liabilities (6) (39)
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 0 0
Net Amount $ (6) $ (39)