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

The number of finance leases.

No definition available.

Name:	opc_NumberOfFinanceLeases
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

The percentage of undivided interest in property, plant and equipment.

No definition available.

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

Term of lessee's finance lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-3

Name:	us-gaap_LesseeFinanceLeaseRenewalTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	opc_LeaseArrangementsAxis=opc_FinanceLeaseAgreementsWithTermsThroughDecember312027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	opc_LeaseArrangementsAxis=opc_FinanceLeaseAgreementsWithTermsThroughJune302031Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	opc_LeaseArrangementsAxis=opc_OperatingLeaseAgreementsWithTermsThroughFebruary2042Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: