Investments - Rocky Mountain transactions (Details) - Rocky Mountain $ in Thousands	2 Months Ended	12 Months Ended
Jan. 31, 1997 investor trust lease	Dec. 31, 2012 USD ($) lease installment
Rocky Mountain transactions
Number of long-term lease transactions \| lease	6	6
Percentage of undivided ownership interest	74.61%
Number of separate owner trusts to whom undivided interest was leased \| trust	6
Number of investors in ownership trusts \| investor	3
Term of lease as a percentage of the estimated useful life of the jointly owned utility plant	120.00%
Term of lease	30 years
Number of leases terminated prior to end of lease term \| lease		5
Percentage of leases which remained in place		10.00%
Basic rental payments due		$ 36,555
Purchase option price		112,000
Outstanding loan amount		$ 74,000
Percentage to be purchased under first option if financing cannot be arranged		49.00%
Maximum
Rocky Mountain transactions
Additional term of sublease		16 years
AIG Matched Funding Corp
Rocky Mountain transactions
Fund amount under payment undertaking agreement		$ 74,000
Fund amount under equity funding agreement		$ 37,928
Number of installments available to pay to the owner trust \| installment		5

Represents the number of leases terminated prior to the end of their lease term.

No definition available.

Name:	opc_LeaseTerminationNumberTerminatedPriorToEndOfLeaseTerm
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the additional term of the sublease under the sale and leaseback transaction.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionAdditionalTermOfSubleaseAgreement
Namespace Prefix:	opc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount represents the sale leaseback transaction, funded amount under equity funding agreement.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionFundedAmountUnderEquityFundingAgreement
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount represents the sale leaseback transaction, funded amount under payment undertaking agreement.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionFundedAmountUnderPaymentUndertakingAgreement
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the number of installments available to pay to the owner trust.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionNumberOfInstallmentsAvailableToPayToOwnerTrust
Namespace Prefix:	opc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of investors in the ownership trusts to whom ownership interest was leased under the sale and leaseback transaction.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of long-term lease transactions entered into by the entity under the sale and leaseback transactions.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of separate owner trusts to whom ownership interest was leased under the sale and leaseback transaction.

+ References

No definition available.

+ Details

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

- Definition

Amount represents outstanding loan amount under sale and leaseback transactions.

+ References

No definition available.

+ Details

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

- Definition

Represents percentage of leases which remained in place under the sale and leaseback transaction.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of undivided ownership interest in assets.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage to be purchased under the first option if financing cannot be arranged.

+ References

No definition available.

+ Details

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

- Definition

Represents term of lease under the sale and leaseback transaction.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionTermOfAgreement
Namespace Prefix:	opc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the term of the lease as a percentage of the estimated useful life of the jointly owned utility plant under the sale and leaseback transaction.

+ References

No definition available.

+ Details

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

- Definition

Amount represents fixed purchase option price to purchase the owner trust's undivided interest.

+ References

No definition available.

+ Details

Name:	opc_SaleLeasebackTransactionsFixedPurchaseOptionPrice
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The liability due as of the balance sheet date for leasebacks accounted for using the deposit method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84164451&loc=d3e36991-112694

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77913982&loc=d3e50796-112755

+ Details

Name:	us-gaap_SaleLeasebackTransactionAmountDueUnderFinancingArrangement
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_SaleLeasebackTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PublicUtilityAxis=opc_RockyMountainMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=opc_AigMatchedFundingCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: