Represents the ratio of margins for interest to total interest charges for a given period.

No definition available.

Name:	opc_MarginsForInterestRatio
Namespace Prefix:	opc_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of electric distribution cooperative members.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- Definition

Represents the number of services provided.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

It represents the percentage of member emery requirements supplied.

+ References

No definition available.

+ Details

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

- Definition

Represents the ratio of targeted margins for interest.

+ References

No definition available.

+ Details

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- Definition

Amount, before allowance for credit loss, of right to consideration in exchange for good or service transferred to customer, when right is conditioned on something other than passage of time.

+ References

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-Publisher FASB
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+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Amount of liability for consideration received or receivable from customer which is not included in transaction price, when consideration is expected to be refunded to customer.

+ References

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+ Details

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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

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- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_MajorCustomersAxis=opc_CooperativeMembersMember
Namespace Prefix:
Data Type:	na
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Period Type:

- Details

Name:	srt_ProductOrServiceAxis=opc_CapacityServicesMember
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Period Type:

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Revenue Recognition (Details) $ in Thousands	3 Months Ended		6 Months Ended
Revenue Recognition (Details) $ in Thousands	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) member service	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Revenue Recognition
Number of electric distribution cooperative members \| member			38
Number of services provided \| service			2
Member energy requirements supplied	62.00%	59.00%	59.00%	55.00%
Margins for interest ratio			1.10
Targeted margins for interest ratio			1.14
Refund liability	$ 4,500	$ 5,850	$ 4,500	$ 5,850
Operating revenues	358,127	330,944	734,458	672,618
Members
Revenue Recognition
Operating revenues	357,921	330,768	734,193	672,281
Receivables from contracts with members	158,121		158,121		$ 135,462
Capacity revenues \| Members
Revenue Recognition
Operating revenues	232,431	240,256	488,255	499,649
Energy revenues \| Members
Revenue Recognition
Operating revenues	$ 125,490	$ 90,512	$ 245,938	$ 172,632