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Net realized and unrealized gains and losses on derivative transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commodity contracts: [Abstract]
Electricity	$ 13por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsElectricity	$ 78por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsElectricity	$ 56por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsElectricity
Natural Gas	72por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsNaturalGas	28por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsNaturalGas	19por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsNaturalGas
Realized And Unrealized Losses Net Commodity Contracts, Foreign Currency	$ 0por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsForeignCurrency	$ 1por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsForeignCurrency	$ 0por_RealizedAndUnrealizedGainsAndLossesNetCommodityContractsForeignCurrency

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- Details

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- Definition

Realized and unrealized gains and losses, net on commodity contracts related to electricity not designated as hedging instruments included in purchased power and fuel expense in the statement of income.

+ References

No definition available.

+ Details

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- Definition

Realized And Unrealized Gains And Losses Net Commodity Contracts, Foreign Currency

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No definition available.

+ Details

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- Definition

Realized and unrealized gains and losses, net on commodity contracts related to natural gas not designated as hedging instruments included in purchased power and fuel expense in the statement of income.

+ References

No definition available.

+ Details

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