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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
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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

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