Derivative Instruments - Gas hedges (Details) - Natural gas swaps $ in Thousands, MMBTU in Millions	12 Months Ended
Dec. 31, 2020 USD ($) MMBTU	Dec. 31, 2019 USD ($)
Derivative Instruments
Derivative liabilities \| $	$ 10,248	$ 32,256
Letters of credit \| $	$ 10,248
Notional volume of natural gas derivatives (in MMBTUs)	100.8
2021
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	27.5
2022
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	23.7
2023
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	18.7
2024
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	17.5
2025
Derivative Instruments
Notional volume of natural gas derivatives (in MMBTUs)	13.4

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

+ Details

Name:	us-gaap_AdditionalCollateralAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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

- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	opc_DerivativeSettlementOrMaturityByPeriodAxis=opc_SettlementOrMaturityPeriod2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	opc_DerivativeSettlementOrMaturityByPeriodAxis=opc_SettlementOrMaturityPeriod2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	opc_DerivativeSettlementOrMaturityByPeriodAxis=opc_SettlementOrMaturityPeriod2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	opc_DerivativeSettlementOrMaturityByPeriodAxis=opc_SettlementOrMaturityPeriod2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	opc_DerivativeSettlementOrMaturityByPeriodAxis=opc_SettlementOrMaturityPeriod2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: