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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, 2019
Dec. 31, 2018
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Cash Collateral (Held)/Posted $ (32) $ (33)
Cash Collateral (Held)/Posted 190 287
Gross Amounts of Recognized Assets / Liabilities   104
Derivative Instruments   0
Cash Collateral (Held) / Posted   83
Net Amount   187
Commodity contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets 1,170 1,042
Derivative Instruments (909) (778)
Cash Collateral (Held)/Posted (7) (31)
Net Amount 254 233
Derivative liabilities (1,103) (977)
Derivative Instruments 909 778
Cash Collateral (Held)/Posted 73 114
Net Amount (121) (85)
Gross Amounts of Recognized Assets / Liabilities 67 65
Derivative Instruments 0 0
Cash Collateral (Held) / Posted 66 83
Net Amount $ 133 148
Interest rate contracts    
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid    
Derivative assets   39
Derivative Instruments   0
Cash Collateral (Held)/Posted   0
Net Amount   39
Gross Amounts of Recognized Assets / Liabilities   39
Derivative Instruments   0
Cash Collateral (Held) / Posted   0
Net Amount   $ 39