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

No definition available.

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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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- 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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Data Type:	num:percentItemType
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- 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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Balance Type:	credit
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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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- 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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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Details

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

- Details

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Period Type:

- Details

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Revenue Recognition (Details) $ in Thousands	3 Months Ended		9 Months Ended
Revenue Recognition (Details) $ in Thousands	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) member service	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Revenue Recognition
Number of electric distribution cooperative members \| member			38
Number of services provided \| service			2
Member energy requirements supplied	65.00%	60.00%	61.00%	57.00%
Margins for interest ratio			1.10
Targeted margins for interest ratio			1.14
Refund liability	$ 16,500	$ 21,400	$ 16,500	$ 21,400
Operating revenues	460,822	366,239	1,195,280	1,038,857
Members
Revenue Recognition
Operating revenues	437,240	365,937	1,171,433	1,038,218
Receivables from contracts	156,095		156,095		$ 135,462
Non-Members
Revenue Recognition
Operating revenues	23,582	302	23,847	639
Receivables from contracts	6,562		6,562		$ 0
Capacity revenues \| Members
Revenue Recognition
Operating revenues	228,048	222,187	716,303	721,836
Energy revenues \| Members
Revenue Recognition
Operating revenues	209,192	143,750	455,130	316,382
Energy revenues \| Non-Members
Revenue Recognition
Operating revenues	$ 23,582	$ 302	$ 23,847	$ 639