Debt Instrument, Covenant, Patronage Capital, Minimum

No definition available.

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

- Definition

Debt Instrument, Covenant, Unsecured Indebtedness, Maximum

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Number Of Lenders

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Unsecured Indebtedness Outstanding

+ References

No definition available.

+ Details

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

- Definition

Represents the number of short-term credit facilities availed by the entity for general working capital purposes.

+ References

No definition available.

+ Details

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

- Definition

Retained net margin and cooperative membership fees.

+ References

No definition available.

+ Details

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

- Definition

Schedule of Debt Instruments [Line Items]

+ References

No definition available.

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-SubTopic 10
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=opc_CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_VariableRateDemandObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=opc_JPMorganMember
Namespace Prefix:
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Period Type:

Debt - Credit Facilities (Details)	1 Months Ended
Debt - Credit Facilities (Details)	Sep. 30, 2024 USD ($)	May 31, 2024 USD ($) lender	Dec. 31, 2024 USD ($) facility	Sep. 29, 2024 USD ($)	May 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Schedule of Debt Instruments [Line Items]
Patronage capital and membership fees	$ 1,328,418,000		$ 1,328,418,000			$ 1,257,917,000
Line of Credit \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Maximum borrowing capacity		$ 1,275,000,000			$ 1,210,000,000
Debt instrument, number of lenders \| lender		11
Term of debt		5 years
Line of Credit \| Line of Credit \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Maximum borrowing capacity			$ 1,725,000,000
Number of separate facilities \| facility			4
Letter of credit \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Maximum borrowing capacity			$ 810,000,000
Available borrowing capacity			807,000,000
Letter of credit \| Variable Rate Demand Obligation \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Available borrowing capacity			2,504,000
Commercial paper \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Line of credit, amount outstanding			$ 401,801,000
JPMorgan \| Line of Credit \| Credit Agreement
Schedule of Debt Instruments [Line Items]
Maximum borrowing capacity	200,000,000			$ 350,000,000
Debt instrument, covenant, patronage capital, minimum	900,000,000			$ 750,000,000
Debt instrument, covenant, unsecured indebtedness, maximum	4,000,000,000
Debt instrument, unsecured indebtedness outstanding	$ 401,801,000