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

No definition available.

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- Definition

Estimated Project-Level Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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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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- Definition

Jointly Owned Utility Plant, Additional Construction Contingency, Total

+ References

No definition available.

+ Details

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- Definition

Jointly Owned Utility Plant, Delay Period

+ References

No definition available.

+ Details

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- 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
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- Definition

Jointly Owned Utility Plant, Monthly Delay Cost

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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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

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Data Type:	num:powerItemType
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- Definition

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

+ References

No definition available.

+ Details

Name:	opc_NumberOfAdditionalNuclearGeneratingPlants
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Data Type:	xbrli:integerItemType
Balance Type:	na
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- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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

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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

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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

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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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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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Data Type:	num:percentItemType
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Period Type:	duration

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Electric plant, construction and related agreements - Narrative (Details) $ in Millions		1 Months Ended		6 Months Ended		12 Months Ended
	Mar. 01, 2021 MW	Apr. 30, 2020	Aug. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) unit MW
Vogtle Units No. 3 & No. 4
Public Utility Property Plant and Equipment
Ownership interest (as a percent)				30.00%		30.00%
Total investment in additional Vogtle units				$ 6,000.0		$ 6,000.0
Additional construction costs exceeds EAC			$ 800.0
Estimated construction project-level contingency			$ 240.0
Excess project contingency, total					$ 75.0
Ownership amount of excess project contingency					23.0
Additional construction contingency, total					250.0	375.0
Ownership amount of additional project level contingency					$ 75.0	112.5
Additional construction contingency, construction productivity and field support				10.0
Ownership amount of additional construction contingency, construction productivity and field support				$ 3.0
Workforce reduction, percent		20.00%
Monthly delay cost						55.0
Vogtle Units No. 3 & No. 4 \| Minimum
Public Utility Property Plant and Equipment
COVID related costs						325.0
Ownership amount of COVID related costs						98.0
Vogtle Units No. 3 & No. 4 \| Maximum
Public Utility Property Plant and Equipment
COVID related costs						415.0
Ownership amount of COVID related costs						$ 125.0
Vogtle Units No. 3 & No. 4 \| 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						$ 7,500.0
Vogtle Units No. 3 & No. 4 \| 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 (in megawatts) \| MW						1,100
Vogtle Units No. 3 & No. 4 \| 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
Vogtle Unit Number 3
Public Utility Property Plant and Equipment
Delay period						4 months
Vogtle Unit Number 4
Public Utility Property Plant and Equipment
Delay period						3 months
Monthly delay cost						$ 25.0
Natural Gas Processing Plant \| Subsequent Event
Public Utility Property Plant and Equipment
Generating capacity of each nuclear unit (in megawatts) \| MW	511