Plant Acquisitions - Identifiable Assets Acquired and Liabilities Assumed (Details) - Natural Gas Processing Plant - Baconton $ in Thousands	Dec. 20, 2022 USD ($)
Plant Acquisition [Line Items]
Electric plant in service, net	$ 16,450
Other current assets	323
Other current liabilities	(30)
Total identifiable net assets	$ 16,743

Asset Acquisition, Recognizable Identifiable Assets Acquired and Liabilities Assumed, Electric Plant in Service

No definition available.

Name:	opc_AssetAcquisitionRecognizableIdentifiableAssetsAcquiredAndLiabilitiesAssumedElectricPlantInService
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Asset Acquisition, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Assets, Other

+ References

No definition available.

+ Details

Name:	opc_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsOther
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Asset Acquisition, Recognized Identifiable Assets Acquired And Liabilities Assumed, Current Liabilities, Other

+ References

No definition available.

+ Details

Name:	opc_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Asset Acquisition, Recognized Identifiable Assets Acquired And Liabilities Assumed, Net

+ References

No definition available.

+ Details

Name:	opc_AssetAcquisitionRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479907/805-20-50-5

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PublicUtilityAxis=us-gaap_NaturalGasProcessingPlantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetAcquisitionAxis=opc_BacontonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: