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
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- Definition

Represents the number of electric distribution cooperative members.

+ References

No definition available.

+ Details

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

- 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:	dtr-types: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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Data Type:	xbrli:pureItemType
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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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+ Details

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Data Type:	xbrli:monetaryItemType
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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-Publisher FASB
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-SubTopic 10
-Section 55
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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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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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+ Details

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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
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=opc_NonMembersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=opc_CapacityServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=opc_EnergyServicesMember
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Data Type:	na
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Period Type:

- Details

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- Details

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Revenue Recognition - Additional Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2023 USD ($) member service	Mar. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)
Revenue Recognition
Number of electric distribution cooperative members \| member	38
Number of services provided \| service	2
Member energy requirements supplied	64.00%	57.00%
Margins for interest ratio	1.10
Targeted margins for interest ratio	1.14
Operating revenues	$ 389,453	$ 420,442
Refund liability	0		$ 28,471
Members
Revenue Recognition
Operating revenues	387,653	417,449
Receivables from contracts	126,448		187,401
Non-Members
Revenue Recognition
Operating revenues	1,800	2,993
Capacity revenues \| Members
Revenue Recognition
Operating revenues	242,043	243,291
Capacity revenues \| Non-Members
Revenue Recognition
Operating revenues	763	0
Energy revenues \| Members
Revenue Recognition
Operating revenues	145,610	174,158
Energy revenues \| Non-Members
Revenue Recognition
Operating revenues	1,037	$ 2,993
Sale of Effingham Deferring Members' Output \| Non-Members
Revenue Recognition
Receivables from contracts	820		8,787
Transactions with Affiliated Companies and Investment Income \| Non-Members
Revenue Recognition
Receivables from contracts	$ 35,867		$ 13,834