Purchase Agreements: (Details) - Precedent Agreement - Transcontinental Gas Pipeline Company, LLC (Transco)	Apr. 11, 2014 USD ($)
Purchase Agreements
Base term of an agreement	25 years
Aggregate agreement fixed charge	$ 942,500,000

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.

No definition available.

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