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Accounting for Derivative Instruments and Hedging Activities - Net notional volume buy/sell of open derivative transactions (Details) shares in Millions, T in Millions, MWh in Millions, MMBTU in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 MMBTU MWh T shares	Dec. 31, 2019 MWh MMBTU T shares
Short | Emissions
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton) | T	1	3
Short | Natural Gas
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MMBTU	152	181
Short | Capacity
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MMBTU	1	1
Long | Renewable Energy Certificates
Volumetric Underlying Derivative Transactions
Derivative, non-monetary notional amount (in shares) | shares	1	1
Long | Coal
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, mass (ton) | T	7	10
Long | Power
Volumetric Underlying Derivative Transactions
Derivative, nonmonetary notional amount, energy (in MMBtu and MWh) | MWh	46	38

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- Definition

Derivative, Non-monetary Notional Amount, Other

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- Definition

Volumetric Underlying Derivative Transactions [Abstract]

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- Definition

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

+ References

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+ Details

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- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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