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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) certificates in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Billions | 9 Months Ended | 12 Months Ended |
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Sep. 30, 2020 MMBTU MWh certificates T | Dec. 31, 2019 MMBTU MWh certificates T | Nov. 05, 2020 USD ($)
Interest Rate Swap | Subsequent Event |  |  |
Derivative [Line Items] |  |  |
Derivative, notional amount | $ |  |  | $ 1.6
Long | Emissions |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 1 | 3 |
Long | Renewable Energy Certificates |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, non-monetary notional amount (in shares) | certificates | 3 | 1 |
Long | Coal |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, nonmonetary notional amount, mass (ton) | T | 4 | 10 |
Long | Power |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MWh | (51) | (38) |
Short | Natural Gas |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU | (264) | (181) |
Short | Capacity |  |  |
Volumetric Underlying Derivative Transactions |  |  |
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU | (1) | (1) |

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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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