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Derivatives (Notional Volumes of Commodity Swaps and Forward Contracts Designated for Regulatory Deferral) (Details) - Designated as Hedging Instrument [Member] - Regulatory Deferral Hedge [Member] - Commodity swaps and forwards [Member] bbl in Millions, T in Millions, MMBTU in Millions	12 Months Ended
	Dec. 31, 2017 MMBTU T bbl
Coal [Member] | 2018 [Member}
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | T	1
Coal [Member] | 2019-2021 [Member}
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass | T	1
Fuel [Member] | 2018 [Member}
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	30
Fuel [Member] | 2019-2021 [Member}
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	14
CrudeOilMember | 2018 [Member}
Derivative [Line Items]
DerivativeNonmonetaryNotionalAmountVolume | bbl	0
CrudeOilMember | 2019-2021 [Member}
Derivative [Line Items]
DerivativeNonmonetaryNotionalAmountVolume | bbl	1

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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X

- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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- Definition

Nominal volume used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

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- Details

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