Amount of additional construction costs that exceeds EAC (estimated cost at completion) in the nineteenth VCM report.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Estimated Project-Level Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Definition

Jointly Owned Utility Plant, Additional Construction Contingency, Construction Productivity And Field Support

+ References

No definition available.

+ Details

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

- Definition

Jointly Owned Utility Plant, Additional Construction Contingency, Total

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Jointly Owned Utility Plant, Excess Project Contingency, Total

+ References

No definition available.

+ Details

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

- Definition

Jointly Owned Utility Plant, Ownership Amount Of Additional Construction Contingency, Construction Productivity And Field Support

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Jointly Owned Utility Plant, Ownership Amount Of Additional Project Level Contingency

+ References

No definition available.

+ Details

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

- Definition

Jointly Owned Utility Plant, Ownership Amount Of Excess Project Contingency

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to the project budget.

+ References

No definition available.

+ Details

Name:	opc_JointlyOwnedUtilityPlantTotalProjectBudget
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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

Percentage of costs disallowed by public service commission for recovery.

+ References

No definition available.

+ Details

Name:	opc_PercentageOfCostsDisallowedByPublicServiceCommissionForRecovery
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 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

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

Unusual Or Infrequent Item, Or Both, Ownership Amount Of Total Expected Cost

+ References

No definition available.

+ Details

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

- Definition

Unusual Or Infrequent Item, Or Both, Total Expected Cost

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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=115995737&loc=d3e659983-123033

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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

No definition available.

+ Details

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

- Definition

The number of positions eliminated during the period as a percentage of total positions eliminated during the period in connection with the restructuring plan(s).

+ References

No definition available.

+ Details

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

- Details

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Vogtle Units No. 3 and No. 4 Construction Project - Narrative (Details) - Vogtle Units No. 3 & No. 4 $ in Millions	1 Months Ended		3 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2020	Aug. 31, 2018 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) unit	Dec. 31, 2008 unit MW
Public Utility Property Plant and Equipment
Ownership interest (as a percent)			30.00%		30.00%
Total investment in additional Vogtle units			$ 5,700		$ 5,700
Construction contingency estimate		$ 800
Estimated project-level construction contingency		$ 240
Excess project contingency, total				$ 75
Ownership amount of excess project contingency				23
Additional construction contingency, total				250
Ownership amount of additional project level contingency				$ 75
Additional construction contingency, construction productivity and field support			10
Ownership amount of additional construction contingency, construction productivity and field support			$ 3
Workforce reduction, percent	20.00%
Minimum
Public Utility Property Plant and Equipment
COVID related costs					150
Ownership amount of COVID related costs					45
Maximum
Public Utility Property Plant and Equipment
COVID related costs					250
Ownership amount of COVID related costs					$ 75
Ownership participation agreement
Public Utility Property Plant and Equipment
Number of additional nuclear units \| unit					2
Ownership interest (as a percent)			30.00%		30.00%
Project budget					$ 7,500
Project extension term					1 year
EPC Agreement \| Westinghouse Electric Company LLC and Stone & Webster, Inc.
Public Utility Property Plant and Equipment
Number of nuclear units \| unit						2
Generating capacity of each nuclear unit \| MW						1,100
Services Agreement \| Westinghouse Electric Company LLC and Stone & Webster, Inc.
Public Utility Property Plant and Equipment
Written notice period for termination of agreement					30 days
Global Amendments To Term Sheet
Public Utility Property Plant and Equipment
Percentage of costs disallowed for recovery			6.00%		6.00%
Global Amendments To Term Sheet \| Minimum
Public Utility Property Plant and Equipment
Percentage of ownership approval to change primary construction contractor			90.00%		90.00%