- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to members when right is conditioned on something other than passage of time.

+ References

No definition available.

+ Details

Name:	opc_ContractWithCustomerAssetReceivablesFromMembers
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Revenue for bundled services from customers who are members that receive electrical generation services.

+ References

No definition available.

+ Details

Name:	opc_ElectricGenerationRevenueSalesToMembers
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Revenue for bundled services from customers who are non members that receive electrical generation services.

+ References

No definition available.

+ Details

Name:	opc_ElectricGenerationRevenueSalesToNonMembers
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the period in which the payment is due after being billed.

+ References

No definition available.

+ Details

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

- 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

- Details

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

- Details

Name:	us-gaap_ProductOrServiceAxis=opc_EnergyServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Revenue Recognition - Disaggregation of revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenue Recognition
Sales to Members	$ 373,401	$ 354,144
Sales to non-Members	$ 245	26
Period in which payment is due following billing	1 month
Receivables from members	$ 117,625	126,150	$ 126,211	$ 136,552
Capacity
Revenue Recognition
Sales to Members	240,481	237,432
Sales to non-Members	0	0
Energy
Revenue Recognition
Sales to Members	$ 132,920	$ 116,712