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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, 2018
Dec. 31, 2017
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Cash Collateral (Held)/Posted $ (33) $ (37)
Cash collateral posted in support of energy risk management activities 287 171
Gross Amounts of Recognized Assets / Liabilities 104 103
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 83 61
Net Amount 187 164
Commodity contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 1,042 744
Derivative Instruments (778) (578)
Cash Collateral (Held)/Posted (31) (11)
Net Amount 233 155
Derivative liabilities (977) (674)
Derivative Instruments 778 578
Cash collateral posted in support of energy risk management activities 114 72
Net Amount (85) (24)
Gross Amounts of Recognized Assets / Liabilities 65 70
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 83 61
Net Amount 148 131
Interest rate contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 39 39
Derivative Instruments 0 0
Cash Collateral (Held)/Posted 0 0
Net Amount 39 39
Derivative liabilities   (6)
Derivative Instruments   0
Cash collateral posted in support of energy risk management activities   0
Net Amount   (6)
Gross Amounts of Recognized Assets / Liabilities 39 33
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 0 0
Net Amount $ 39 $ 33