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Derivative Instruments and Hedging Activities - Volumetric Underlying Derivative Transactions (Details) $ in Millions		9 Months Ended
	Dec. 31, 2019 USD ($) MWh MMBTU	Sep. 30, 2020 USD ($) MMBTU MWh
Natural Gas | Long
Derivatives, Fair Value [Line Items]
Derivative nonmonetary notional amount (energy measure) | MMBTU	2	1
Power | Short
Derivatives, Fair Value [Line Items]
Derivative nonmonetary notional amount (energy measure) | MWh	2	2
Interest | Long
Derivatives, Fair Value [Line Items]
Derivative, notional amount | $	$ 1,788	$ 1,769

X

- Definition

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

+ References

No definition available.

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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=109980867&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=109980867&loc=SL5579245-113959

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- Definition

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+ References

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