Revenue Recognition (Details)	3 Months Ended
Mar. 31, 2020 USD ($) service member	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Revenue Recognition
Number of electric distribution cooperative members \| member	38
Number of services provided \| service	2
Energy supplied to members, percent	50.00%	52.00%
Margins for interest ratio	1.10
Targeted margins for interest ratio	1.14
Refund liability	$ 0	$ 0
Operating revenues	$ 341,674,000	$ 356,600,000
Member energy requirements supplied	50.00%	52.00%
Members
Revenue Recognition
Operating revenues	$ 341,513,000	$ 356,470,000
Receivables from contracts with members	114,660,000		$ 142,640,000
Capacity revenues \| Members
Revenue Recognition
Operating revenues	259,393,000	245,986,000
Energy revenues \| Members
Revenue Recognition
Operating revenues	$ 82,120,000	$ 110,484,000

Energy Supplied to Members, Percent

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

Name:	opc_NumberOfElectricDistributionCooperativeMembers
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of services provided.

+ References

No definition available.

+ Details

Name:	opc_NumberOfServicesProvided
Namespace Prefix:	opc_
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

Name:	opc_PercentageOfMembersEnergyRequirementsSupplied
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

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

- 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130531-203044

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130533-203044

+ Details

Name:	us-gaap_ContractWithCustomerAssetGross
Namespace Prefix:	us-gaap_
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 55
-Paragraph 27
-URI http://asc.fasb.org/extlink&oid=108792157&loc=SL49130611-203046-203046

+ Details

Name:	us-gaap_ContractWithCustomerRefundLiability
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_DisaggregationOfRevenueLineItems
Namespace Prefix:	us-gaap_
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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130543-203045

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130539-203045

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130545-203045

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
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_ProductOrServiceAxis=opc_CapacityServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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