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Derivatives (Notional Volumes of Foreign Exchange Swaps and Forward Contracts Related to Commodity Contracts) (Details) - Regulatory Deferral Hedge [Member] - Foreign Exchange Swaps and Forward Contracts [Member] - Fuel [Member] $ in Millions	Dec. 31, 2017 USD ($)
2018 [Member}
Derivative [Line Items]
Derivative, Notional Amount	$ 144
Derivative, Average Forward Exchange Rate	1.1061
Derivative, Percent of USD Required to Purchase	79.00%
2019-2021 [Member}
Derivative [Line Items]
Derivative, Notional Amount	$ 156
Derivative, Average Forward Exchange Rate	1.2001
Derivative, Percent of USD Required to Purchase	40.00%

X

- Definition

Percent of USD fuel required to purchase.

+ References

No definition available.

+ Details

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

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- Definition

Average contractual rate at which a foreign currency can be purchased or sold.

+ References

No definition available.

+ Details

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- 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

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- Details

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- Details

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