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Derivative Accounting - Outstanding Gross Notional Amounts Outstanding (Details) | 12 Months Ended
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Dec. 31, 2017 MWh Bcf | Dec. 31, 2016 MWh Bcf
Outstanding gross notional amount of derivatives |  |
Power (in MWh) | MWh | 583,000 | 1,314,000
Gas (in bcf) | Bcf | 240 | 194

X
- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
Name: | us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix: | us-gaap_
Data Type: | num:energyItemType
Balance Type: | na
Period Type: | duration

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- Definition Nominal volume used to calculate payments on a derivative instrument. + References No definition available. + Details
Name: | us-gaap_DerivativeNonmonetaryNotionalAmountVolume
Namespace Prefix: | us-gaap_
Data Type: | num:volumeItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_NotionalAmountOfDerivativesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration