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Accounting for Derivative Instruments and Hedging Activities - Net Notional Volume Buy/Sell of Open Derivative Transactions (Details) shares in Millions, T in Millions, MWh in Millions, MMBTU in Millions | 12 Months Ended
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Dec. 31, 2020 MWh MMBTU T shares | Dec. 31, 2019 MWh MMBTU T shares
Long | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 1 | 3
Long | Renewables Energy Certificates |  |
Derivative [Line Items] |  |
Derivative, non-monetary notional amount (in shares) | shares | 5 | 1
Long | Coal |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 2 | 10
Long | Power |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 57 | 38
Short | Natural Gas |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU | 286 | 181
Short | Capacity |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 1 | 1

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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