Eligible Project Costs, Percent

No definition available.

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- Definition

The amount represents the guarantee obligations maximum exposure received.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of interest to be accrued and payable prior to a specified period, which will be reflected as additional borrowings under the Facility.

+ References

No definition available.

+ Details

Name:	opc_InterestAccruedAndPayablePriorToSpecifiedPeriodReflectedAsAdditionalBorrowingsUnderFacility
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility designated for capitalized interest.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of future advance promissory notes.

+ References

No definition available.

+ Details

Name:	opc_NumberOfFutureAdvancePromissoryNotes
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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The required percentage of ownership to continue construction.

+ References

No definition available.

+ Details

Name:	opc_OwnershipPercentageToContinueConstruction
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the period of cessation of construction activities which would result in prepayment of outstanding principal.

+ References

No definition available.

+ Details

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

- Definition

Represents the period of failure to fund operation and maintenance expenses which would result in prepayment of outstanding principal.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

No definition available.

+ Details

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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

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=opc_DebtFromFederalFinancingBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_VariableRateAxis=opc_USTreasurySecuritiesCurrentYieldMember
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Debt - Department of Energy Loan Guarantee (Details)	6 Months Ended
Debt - Department of Energy Loan Guarantee (Details)	Jun. 30, 2020 USD ($)	Mar. 22, 2019 USD ($)	Dec. 31, 2017 USD ($)	Feb. 20, 2014 USD ($) note
Vogtle Units No. 3 & No. 4
Debt
Guarantee Obligations Maximum Exposure Received			$ 1,104,000,000
Loan Guarantee Agreement
Debt
Term of debt	5 years
Period of cessation of construction activities which would result in prepayment of outstanding principal	12 months
Period of failure to fund operation and maintenance expenses which would result in prepayment of outstanding principal	12 months
Long-term debt
Debt
Ownership interests voting required to continue construction (as a percent)	90.00%
Long-term debt \| Department of Energy guarantee
Debt
Aggregate borrowings including capitalized interest	$ 3,457,348,000
Long-term debt \| FFB
Debt
Number of future advance promissory notes \| note				2
Maximum borrowing capacity	4,633,028,088	$ 4,676,749,167		$ 3,057,069,461
Capitalized interest				335,471,604
Maximum borrowing capacity designated for capitalized interest				$ 43,721,079
Eligible project costs, percent		70.00%
Long-term debt \| FFB \| Department of Energy guarantee
Debt
Aggregate borrowings including capitalized interest	$ 444,000,000	$ 1,619,679,706
Long-term debt \| FFB \| US Treasury Securities, Current Yield
Debt
Spread on variable rate (as a percent)	0.375%