The amount of additional construction costs.

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

An estimate of the additional capital costs required to complete a construction project in-process.

+ References

No definition available.

+ Details

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- Definition

Estimated Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Estimated Project-Level Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

Name:	opc_EstimatedProjectLevelConstructionContingency
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- 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

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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

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

Percentage of construction costs to be responsible by each Co-owner.

+ References

No definition available.

+ Details

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

- Definition

Percentage of cost savings entitled to each Co-owner.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
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

Percentage of number of Co-owners releasing interim project reports.

+ References

No definition available.

+ Details

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

- 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

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Data Type:	num:percentItemType
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- Definition

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

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of potential additional costs to incurred if additional construction costs exceeds EAC (estimated cost at completion).

+ References

No definition available.

+ Details

Name:	opc_PotentialAdditionalCosts
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

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

- Definition

Represents the project extension term.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
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- Definition

The amount of reduction in additional construction costs.

+ References

No definition available.

+ Details

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- 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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Balance Type:	na
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- 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

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Balance Type:	credit
Period Type:	instant

- 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

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Balance Type:	na
Period Type:	duration

- 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

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- 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://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(22))
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-Name Accounting Standards Codification
-Topic 942
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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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

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

- Definition

The total number of new claims filed pertaining to a loss contingency during the period.

+ References

Reference 1: 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=82911808&loc=d3e14435-108349

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Data Type:	xbrli:integerItemType
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- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

No definition available.

+ Details

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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

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Electric plant, construction and related agreements: - Construction (Details)		1 Months Ended	12 Months Ended
	Mar. 15, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2008 item MW	Sep. 30, 2017 USD ($)	Sep. 28, 2017 USD ($)	Feb. 20, 2014 USD ($)
Long-term debt
Electric plant, construction and related agreements
Ownership interests voting required to continue construction (as a percent)			90.00%
Maximum borrowing capacity						$ 4,676,749,167
Long-term debt \| FFB
Electric plant, construction and related agreements
Maximum borrowing capacity			$ 4,676,749,167		$ 1,619,679,706		$ 3,057,069,461
Aggregate Department of Energy-guaranteed borrowings			$ 1,794,723,000
Long-term debt \| FFB \| Maximum \| Subsequent Event
Electric plant, construction and related agreements
Advances received on loans	$ 585,000,000
Global Amendments to Term Sheet \| Long-term debt
Electric plant, construction and related agreements
Term of credit facility (in years)			5 years
Vogtle Units No. 3 & No. 4
Electric plant, construction and related agreements
Number of additional nuclear units \| item			2
Ownership interest (as a percent)			30.00%
Number of petitions filed by parties			2
Estimated additional costs of completion			$ 450,000,000
Estimated construction project-level contingency			240,000,000
Capital market debt issuances			$ 1,887,000,000
Vogtle Units No. 3 & No. 4 \| Long-term debt \| FFB
Electric plant, construction and related agreements
Term of credit facility (in years)			5 years
Vogtle Units No. 3 & No. 4 \| Ownership participation agreement
Electric plant, construction and related agreements
Project budget		$ 7,500,000,000
Estimated construction contingency		$ 500,000,000
Total investment in additional Vogtle units			$ 3,900,000,000
Vogtle Units No. 3 & No. 4 \| Ownership participation agreement \| Occurrence of certain adverse events
Electric plant, construction and related agreements
Project extension term			1 year
Vogtle Units No. 3 & No. 4 \| Ownership participation agreement \| Minimum \| Occurrence of certain adverse events
Electric plant, construction and related agreements
Ownership interests voting required to continue construction (as a percent)			90.00%
Vogtle Units No. 3 & No. 4 \| EPC Agreement \| Westinghouse Electric Company LLC and Stone & Webster, Inc.
Electric plant, construction and related agreements
Number of nuclear units \| item				2
Generating capacity of each nuclear unit \| MW				1,100
Vogtle Units No. 3 & No. 4 \| Services Agreement \| Westinghouse Electric Company LLC and Stone & Webster, Inc.
Electric plant, construction and related agreements
Written notice period for termination of agreement			30 days
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet
Electric plant, construction and related agreements
Project budget			$ 8,400,000,000
Additional construction costs			$ 800,000,000
Percentage of number of Co-owners			67.00%
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Occurrence of certain adverse events
Electric plant, construction and related agreements
Percentage of costs disallowed for recovery			6.00%
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			24.50%
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			19.00%
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Minimum \| Occurrence of certain adverse events
Electric plant, construction and related agreements
Ownership interests voting required to continue construction (as a percent)			90.00%
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 No. 3 & No. 4 \| Global Amendments to Term Sheet \| Minimum \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Additional construction costs exceeds EAC			$ 800,000,000
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Minimum \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Additional construction costs exceeds EAC			1,600,000,000
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Minimum \| EAC exceeds more than $2.1 billion
Electric plant, construction and related agreements
Additional construction costs exceeds EAC			2,100,000,000
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Maximum \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Additional construction costs exceeds EAC			1,600,000,000
Construction costs savings			44,000,000
Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Maximum \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Additional construction costs exceeds EAC			2,100,000,000
Construction costs savings			55,000,000
Co-owners \| Vogtle Units No. 3 & No. 4
Electric plant, construction and related agreements
Estimated additional costs of completion			1,500,000,000
Estimated construction project-level contingency			$ 800,000,000
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			55.70%
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			65.70%
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds more than $2.1 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			100.00%
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Actual cost at completion less than EAC
Electric plant, construction and related agreements
Percentage of cost savings entitled			60.70%
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Maximum \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Potential Additional Costs			$ 80,000,000
Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| Maximum \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Potential Additional Costs			$ 100,000,000
Co-owners excluding Georgia Power \| Global Amendments to Term Sheet \| Actual cost at completion less than EAC
Electric plant, construction and related agreements
Percentage of cost savings entitled			39.30%
Co-owners excluding Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $800 million to $1.6 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			44.30%
Co-owners excluding Georgia Power \| Vogtle Units No. 3 & No. 4 \| Global Amendments to Term Sheet \| EAC exceeds by $1.6 billion to $2.1 billion
Electric plant, construction and related agreements
Percentage of construction costs to be responsible by each Co-owner			34.30%