Commitments (Details) $ in Thousands	12 Months Ended
Dec. 31, 2024 USD ($)
Finance and Operating Leases
2025	$ 17,337
2026	17,045
2027	16,768
2028	4,761
2029	3,315
Thereafter	5,302
Coal
Long-term Purchase Commitment [Line Items]
2025	21,349
2026	14,321
2027	7,189
2028	0
2029	0
Thereafter	0
Nuclear Fuel
Long-term Purchase Commitment [Line Items]
2025	91,650
2026	50,010
2027	30,930
2028	38,280
2029	29,730
Thereafter	48,810
Gas Transportation
Long-term Purchase Commitment [Line Items]
2025	68,568
2026	72,032
2027	72,328
2028	93,565
2029	217,694
Thereafter	3,556,334
Maintenance Agreements
Long-term Purchase Commitment [Line Items]
Cancellation obligation	66,866
2025	96,851
2026	15,019
2027	2,508
2028	51,743
2029	56,014
Thereafter	223,583
Asset Retirement Obligations
Long-term Purchase Commitment [Line Items]
2025	56,811
2026	49,386
2027	43,935
2028	30,331
2029	42,380
Thereafter	$ 10,639,479
Scherer Unit No. 2
Long-term Purchase Commitment [Line Items]
Ownership interest (as a percent)	60.00%

The amount of convenience penalties which are to be paid if theres is any cancellation of contractual obligation.

No definition available.

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

- Definition

Amount of lessee's undiscounted obligation for lease payments for finance lease and operating lease, due after fifth fiscal year following latest fiscal year.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Lessee, Finance Lease and Operating Lease, Liability, Payments, Due Next Twelve Months

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Amount of lessee's undiscounted obligation for lease payments for finance lease and operating lease, due in fifth fiscal year following latest fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Amount of lessee's undiscounted obligation for lease payments for finance lease and operating lease, due in fourth fiscal year following latest fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Amount of lessee's undiscounted obligation for lease payments for finance lease and operating lease, due in third fiscal year following latest fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Amount of lessee's undiscounted obligation for lease payments for finance lease and operating lease, due in second fiscal year following latest fiscal year.

+ References

No definition available.

+ Details

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

- Definition

n/a

+ References

No definition available.

+ Details

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Namespace Prefix:	opc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 360
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479070/980-360-S99-1

+ Details

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Data Type:	dtr-types:percentItemType
Balance Type:	na
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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Balance Type:	na
Period Type:	duration

- Definition

Amount of purchase arrangement to be paid after fifth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in fifth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in fourth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in second fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in third fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
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- Details

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- Details

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- Details

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- Details

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- Details

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