Represents the estimated aggregate costs to suspend or cancel the project site.

No definition available.

Name:	opc_EstimatedAggregateCancellationCostsOfSite
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of guarantee obligations received in installments.

+ References

No definition available.

+ Details

Name:	opc_GuaranteeObligationsReceived
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase in project budget.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to the period of written notice to be sent in the event of non-renewal of letter of credit.

+ References

No definition available.

+ Details

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

- Definition

The percentage of contract price provided as liquidated damages under the agreement.

+ References

No definition available.

+ Details

Name:	opc_LossContingencyPercentageOfContractPriceLiquidatedDamages
Namespace Prefix:	opc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the total nominally rated generating capacity per nuclear unit.

+ References

No definition available.

+ Details

Name:	opc_NominallyRatedGeneratingCapacity
Namespace Prefix:	opc_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the additional number of units constructed in connection with the generation of nuclear energy.

+ References

No definition available.

+ Details

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

- Definition

The number of guarantee obligation installments received.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of units constructed in connection with the generation of nuclear energy.

+ References

No definition available.

+ Details

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

- 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

The percentage of ownership approval required for material amendments to the Services Agreement or agreements with the primary construction contractor or Southern Nuclear.

+ References

No definition available.

+ Details

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

- Definition

The percentage of ownership approval required to change the primary construction contractor under the amended agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to the project budget.

+ References

No definition available.

+ Details

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

- Definition

Represents the project extension term.

+ References

No definition available.

+ Details

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

- Definition

Information pertaining to the recovery of the the Guarantee Obligations assumed, represented as a percentage.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to the period of written notice to be sent to terminate the agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the total investment made in additional vogtle units.

+ References

No definition available.

+ Details

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

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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

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

- Definition

Amount of loss contingency liability.

+ References

+ Details

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

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

- Definition

Amount of stockholders' equity (deficit), net of receivables from officers, directors, owners, and affiliates of the entity, attributable to both the parent and noncontrolling interests. Amount excludes temporary equity. Alternate caption for the concept is permanent equity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4I
-URI http://asc.fasb.org/extlink&oid=65897068&loc=SL4590271-111686

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4568740-111683

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=68079674&loc=SL4568447-111683

+ Details

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

- Details

Name:	us-gaap_PublicUtilityAxis=opc_VogtleUnitsNumber3AndNumber4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LienCategoryAxis=opc_RemovalOfSubcontractorLiensMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=opc_WestinghouseElectricCompanyLlcAndStoneWebsterInc.AndWectecStaffingServicesLlcMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=opc_OwnershipParticipationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=opc_CertainAdverseEventsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=opc_EngineeringProcurementAndConstructionAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=opc_WestinghouseElectricCompanyLlcAndStoneWebsterInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=opc_EventOfAbandonmentOfWorkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=opc_WestinghouseElectricCompanyLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=opc_GuaranteeSettlementAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=opc_CoOwnersAndToshibaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=opc_AmendedAndRestatedServicesAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=opc_ToshibaCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Vogtle Units No. 3 and No. 4 Construction Project (Details) $ in Millions				1 Months Ended	2 Months Ended	9 Months Ended	12 Months Ended
	Nov. 02, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 09, 2017 USD ($)	Jan. 31, 2016 USD ($)	Nov. 30, 2017 USD ($) installment	Sep. 30, 2017 USD ($) item	Dec. 31, 2008 USD ($) item MW
Vogtle Units Number 3 And Number 4
Vogtle Units No. 3 and No. 4 Construction Project
Current investment		$ 3,900.0				$ 3,900.0
Vogtle Units Number 3 And Number 4 \| Removal of Subcontractor liens
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages		115.0
Aggregate liability paid or accrued		340.0				$ 340.0
Vogtle Units Number 3 And Number 4 \| Removal of Subcontractor liens \| The Contractor and WECTEC Staffing Services LLC
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages		$ 386.0
Vogtle Units Number 3 And Number 4 \| Ownership participation agreement
Vogtle Units No. 3 and No. 4 Construction Project
Number of additional nuclear units \| item						2
Ownership interest (as a percent)		30.00%				30.00%
Vogtle Units Number 3 And Number 4 \| Ownership participation agreement \| Minimum \| Occurrence of certain adverse events
Vogtle Units No. 3 and No. 4 Construction Project
Ownership interests voting required to continue construction (as a percent)	90.00%
Increase in project budget	$ 1,000.0
Project extension term	1 year
Ownership approval to change primary construction contractor (as a percent)	90.00%
Ownership approval required for material amendments (as a percent)	67.00%
Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the EPC Agreement) \| Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor)
Vogtle Units No. 3 and No. 4 Construction Project
Number of nuclear units \| item							2
Generating capacity of each nuclear unit \| MW							1,100
Project budget		$ 7,000.0				$ 7,000.0
Estimated aggregate cancellation costs of site						$ 230.0
Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the EPC Agreement) \| Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor) \| In the event of abandonment of work
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages, as a percent of contract price							40.00%
Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the EPC Agreement) \| Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor) \| Maximum
Vogtle Units No. 3 and No. 4 Construction Project
Liquidated damages, as a percent of contract price							10.00%
Liquidated damages							$ 920.0
Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the EPC Agreement) \| Westinghouse Electric Company LLC
Vogtle Units No. 3 and No. 4 Construction Project
Written notice period for non-renewal of letter of credit				60 days
Vogtle Units Number 3 And Number 4 \| Guarantee Settlement Agreement
Vogtle Units No. 3 and No. 4 Construction Project
Guarantee obligations received in installments					$ 113.3
Recovery of guarantee obligations assumed (as a percent)		100.00%				100.00%
Vogtle Units Number 3 And Number 4 \| Guarantee Settlement Agreement \| In the event of abandonment of work
Vogtle Units No. 3 and No. 4 Construction Project
Guarantee obligations, maximum exposure			$ 1,100.0
Vogtle Units Number 3 And Number 4 \| Guarantee Settlement Agreement \| The Co-owners and Toshiba
Vogtle Units No. 3 and No. 4 Construction Project
Number of installments received \| installment					2
Guarantee obligations received in installments					$ 377.5
Vogtle Units Number 3 And Number 4 \| Guarantee Settlement Agreement \| The Co-owners and Toshiba \| In the event of abandonment of work
Vogtle Units No. 3 and No. 4 Construction Project
Guarantee obligations, maximum exposure			$ 3,680.0
Vogtle Units Number 3 And Number 4 \| Amended and Restated Services Agreement \| Westinghouse Electric Company LLC and Stone & Webster, Inc. (collectively, the Contractor)
Vogtle Units No. 3 and No. 4 Construction Project
Written notice period for termination of agreement			30 days
Toshiba Corporation
Vogtle Units No. 3 and No. 4 Construction Project
Shareholder's equity negative balance		$ 5,500.0				$ 5,500.0
Toshiba Corporation \| Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the EPC Agreement) \| Westinghouse Electric Company LLC
Vogtle Units No. 3 and No. 4 Construction Project
Letters of credit				$ 920.0