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Accounting for Derivative Instruments and Hedging Activities - Net Notional Volume Buy/Sell of Open Derivative Transactions (Details) shares in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($) MWh MMBTU T shares bbl | Dec. 31, 2018 USD ($) MWh MMBTU T shares bbl
Long | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | 3 |
Long | Renewables Energy Certificates |  |
Derivative [Line Items] |  |
Derivative, non-monetary notional amount (in shares) | shares | 1 | 1
Long | Coal |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) | 10 | 13
Long | Oil |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, volume (in barrels) | bbl | 0 | 1
Long | Power |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MWh | 38 | 1
Long | Interest |  |
Derivative [Line Items] |  |
Derivative, Notional Amount (in usd) | $ | $ 0 | $ 1,000
Short | Emissions |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, mass (ton) |  | 2
Short | Natural Gas |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount, energy measure in (millions of btu) and (megawatt hours) | MMBTU | 181 | 330
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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