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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) certificate in Millions, bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) MMBTU MWh certificate T bbl	Dec. 31, 2020 USD ($) MMBTU MWh certificate T bbl
Long | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton) | T	1	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	3	2
Long | Oil
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount (in barrels) | bbl	1	0
Long | Power
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MWh	185	57
Long | Foreign Exchange
Volumetric Underlying Derivative Transactions
Derivative, notional amount | $	$ 218	$ 0
Short | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU	663	286
Short | Capacity
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy measure (MMBtu/MW/Day) | MMBTU	0	1

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Volumetric Underlying Derivative Transactions [Abstract]

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Nominal energy measure used to calculate payments on a derivative instrument.

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Nominal volume used to calculate payments on a derivative instrument.

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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=125515794&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=125515794&loc=SL5579245-113959

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