Name:	opc_LossContingencyDamagesSoughtPercentageOfCooperativesTotalBookedPatronageCapital
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the number of current consumer members.

No definition available.

Name:	opc_LossContingencyNumberOfCurrentConsumerMembers
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of defendant cooperative members named in a legal action during the period.

No definition available.

Name:	opc_LossContingencyNumberOfDefendantMembers
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of members excluded in a legal action against whom defendant class action is certified during the period.

No definition available.

Name:	opc_LossContingencyNumberOfExcludedMembersAgainstWhomDefendantClassActionsCertified
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of former consumer members.

No definition available.

Name:	opc_LossContingencyNumberOfFormerConsumerMembers
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of members in which the consumer members filing the lawsuit belonged.

No definition available.

Name:	opc_LossContingencyNumberOfMembersInWhichFormerConsumerMembersBelonged
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the number of plaintiffs' classes seeking certification.

No definition available.

Name:	opc_LossContingencyNumberOfPlaintiffsClassesSeekingCertification
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the period of revolving schedule to adopt policies to periodically retire the patronage capital from the date of its allocation in PnYnMnDTnHnMnS format, for example, P1Y5M13D represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	opc_LossContingencyPeriodOfRevolvingScheduleToAdoptPoliciesToPeriodicallyRetirePatronageCapital
Namespace Prefix:	opc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Represents the period of revolving schedule to retire the patronage capital from the date of its allocation in PnYnMnDTnHnMnS format, for example, P1Y5M13D represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	opc_LossContingencyPeriodOfRevolvingScheduleToRetirePatronageCapital
Namespace Prefix:	opc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Represents the number of electric distribution cooperative members.

No definition available.

Name:	opc_NumberOfElectricDistributionCooperativeMembers
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.