Price Risk Management Counterparties Representing 10% or More of Assets and Liabilities from price risk management activities (Details)	Sep. 30, 2017	Dec. 31, 2016
Assets from price risk management activities:
Counterparty A	53.00%	22.00%
Counterparty B	3.00%	17.00%
Counterparty C	1.00%	12.00%
Counterparty D	15.00%	0.00%
Counterparty E	10.00%	0.00%
Total	82.00%	51.00%
Liabilities from price risk management activities:
Counterparty F	72.00%	66.00%
Total	72.00%	66.00%

Assets from price risk management activities: [Abstract]

No definition available.

Name:	por_AssetsFromPriceRiskManagementActivitiesAbstract
Namespace Prefix:	por_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents aggregate of individually significant counterparties of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_ConcentrationOfRiskDerivativeInstrumentsAssets
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents aggregate of individually significant counterparties of total liabilities arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_ConcentrationOfRiskDerivativeInstrumentsLiabilities
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individually significant counterparty of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_CounterpartyA
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individually significant counterparty of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_CounterpartyB
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individually significant counterparty of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_CounterpartyC
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individually significant counterparty of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_CounterpartyD
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Counterparty D for PRM Liab

+ References

No definition available.

+ Details

Name:	por_CounterpartyDforPRMLiab
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Individually significant counterparty of total assets arising from derivative instruments from price risk management activities.

+ References

No definition available.

+ Details

Name:	por_CounterpartyE
Namespace Prefix:	por_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Liabilities from price risk management activities: [Abstract]

+ References

No definition available.

+ Details

Name:	por_LiabilitiesFromPriceRiskManagementActivitiesAbstract
Namespace Prefix:	por_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration