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

No definition available.

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

An estimate of the additional allowance for funds used during construction costs required to complete a construction project in-process.

+ References

No definition available.

+ Details

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Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Estimated Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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Balance Type:	debit
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- Definition

Estimated Increase In Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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

- Definition

Estimated Project-Level Cost To Complete, Construction Contingency Estimate

+ References

No definition available.

+ Details

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

The increase in construction project budget.

+ References

No definition available.

+ Details

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

- Definition

The amount of conditional commitment issued by the guarantor under the Loan Guarantee Agreement.

+ References

No definition available.

+ Details

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

- Definition

Represents the total nominally rated generating capacity per nuclear unit.

+ References

No definition available.

+ Details

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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

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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
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- Definition

The number of subcontractors participating in a construction project.

+ References

No definition available.

+ Details

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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

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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

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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

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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

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

- Definition

Represents information pertaining to the maximum range of the project budget.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to the minimum range of the project budget.

+ References

No definition available.

+ Details

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Namespace Prefix:	opc_
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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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

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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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Namespace Prefix:	opc_
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

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-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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Data Type:	xbrli:monetaryItemType
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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-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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Namespace Prefix:	us-gaap_
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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Namespace Prefix:	us-gaap_
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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- 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
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Period Type:	duration

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- Details

Name:	us-gaap_TypeOfArrangementAxis=opc_AmendedAndRestatedServicesAgreementMember
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Vogtle Units No. 3 and No. 4 Construction Project (Details)	1 Months Ended	6 Months Ended	12 Months Ended
	Nov. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Dec. 31, 2008 item MW	Dec. 31, 2018 USD ($)	Sep. 28, 2017 USD ($)
Loan Guarantee Agreement
Electric plant, construction and related agreements
Aggregate Department of Energy-guaranteed borrowings		$ 1,764,658,000
Term of credit facility (in years)		5 years
Loan Guarantee Agreement \| Maximum
Electric plant, construction and related agreements
Aggregate borrowings		$ 3,057,069,461
Conditional commitment issued by guarantor under the Loan Guarantee Agreement					$ 1,619,679,706
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
Joint Ownership Agreements \| Minimum
Electric plant, construction and related agreements
Ownership interests voting required to continue construction (as a percent)	90.00%
Increase in project budget	$ 1,000,000,000
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
Electric plant, construction and related agreements
Number of petitions filed by parties \| item		2
Estimated additional costs of completion		$ 450,000,000
Number of subcontractors \| item		60
Estimated increase in construction contingency		$ 240,000,000
Estimated construction contingency		500,000,000
Project budget		7,000,000,000
Total investment in additional vogtle units		3,396,731,000
Capital market debt issuances		1,387,000,000
Vogtle Units Number 3 And Number 4 \| Extension of the project schedule
Electric plant, construction and related agreements
Estimate of additional base capital monthly costs		30,000,000
Estimate of additional allowance for funds used during construction monthly per unit costs		$ 12,500,000
Vogtle Units Number 3 And Number 4 \| Minimum \| Forecast
Electric plant, construction and related agreements
Project budget estimated minimum				$ 7,250,000,000
Vogtle Units Number 3 And Number 4 \| Maximum \| Forecast
Electric plant, construction and related agreements
Project budget estimated maximum				$ 7,500,000,000
Vogtle Units Number 3 And Number 4 \| Ownership participation agreement
Electric plant, construction and related agreements
Number of additional nuclear units \| item		2
Ownership interest (as a percent)		30.00%
Vogtle Units Number 3 And Number 4 \| Engineering, Procurement and Construction Agreement (the 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
Co-owners \| Vogtle Units Number 3 And Number 4
Electric plant, construction and related agreements
Estimated additional costs of completion		$ 1,500,000,000
Estimated construction project-level contingency		$ 800,000,000