v3.25.1

Summary of significant accounting policies - Patronage capital and membership fees and Margin policy (Details)	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2023	Dec. 31, 2022
Patronage capital and membership fees
Membership fees	$ 190
Minimum equity as a percentage of total long-term debt and equities for distributions of patronage capital	20.00%
Maximum percentage of aggregate net margins in which specified percentage of total long-term debt and equities cannot exceed on or after distributions expended	35.00%
Minimum equity as a percentage of total long-term debt and equities after distributions of patronage capital	30.00%
Margin policy
Minimum margins for interest ratio under the first mortgage indenture	1.10	1.10	1.10
Achieved margins for interest ratio	1.14	1.14	1.14

Represents the ratio of achieved margins for interest.

+ References

No definition available.

+ Details

Name:	opc_AchievedMarginsForInterestRatio
Namespace Prefix:	opc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	opc_InterestRatioMarginsAbstract
Namespace Prefix:	opc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the cooperative membership fees paid by the members of the entity.

+ References

No definition available.

+ Details

Name:	opc_MembershipFees
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the minimum margins to interest ratio required to be maintained under first mortgage indenture by the entity.

+ References

No definition available.

+ Details

Name:	opc_MinimumMarginsToInterestRatio
Namespace Prefix:	opc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	opc_PatronageCapitalAndMembershipFeesAbstract
Namespace Prefix:	opc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum percentage of aggregate net margins in which specified percentage of total long-term debt and equities cannot exceed on or after distributions expended under the patronage capital covenant.

+ References

No definition available.

+ Details

Name:	opc_PatronageCapitalDistributionCovenantAggregateAmountOfDistributionAsPercentageOfAggregateNetMarginsEarnedWhenEquityReachesSpecifiedPercentageOfAggregateLongtermDebtAndEquities
Namespace Prefix:	opc_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum equity as percentage of total long-term debt and equities for distributions of patronage capital at end of immediately preceding fiscal quarter as required under first mortgage indenture.

+ References

No definition available.

+ Details

Name:	opc_PatronageCapitalDistributionCovenantMinimumEquityAsPercentageOfAggregateLongTermDebtAndEquitiesAtEndOfImmediatelyPrecedingFiscalQuarter
Namespace Prefix:	opc_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum equity as percentage of total long-term debt and equities for distributions of patronage capital after giving effect to distribution at the end of immediately preceding fiscal quarter as required under first mortgage indenture.

+ References

No definition available.

+ Details

Name:	opc_PatronageCapitalDistributionCovenantMinimumEquityAsPercentageOfAggregateLongTermDebtAndEquitiesAtEndOfImmediatelyPrecedingFiscalQuarterAfterGivingEffectToDistribution
Namespace Prefix:	opc_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

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