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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) certificate in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 3 Months Ended | 12 Months Ended
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Mar. 31, 2021 USD ($) MMBTU MWh certificate T | Dec. 31, 2020 USD ($) MMBTU MWh certificate T
Long | Emissions |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 0 | 1
Long | Renewable Energy Certificates |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, non-monetary notional amount (in shares) | certificate | 13 | 5
Long | Coal |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 2 | 2
Long | Power |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MWh | 201 | 57
Long | Foreign Exchange |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, notional amount | $ | $ 158 | $ 0
Short | Natural Gas |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU | 605 | 286
Short | Capacity |  |
Volumetric Underlying Derivative Transactions |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU | 0 | 1

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- Definition Volumetric Underlying Derivative Transactions [Abstract] + References No definition available. + Details
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- Definition Nominal or face amount used to calculate payment on derivative. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5580258-113959 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5579245-113959 + Details
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