Purchase agreements (Precedent Agreement, Transcontinental Gas Pipeline Company LLC [Member], USD $)	0 Months Ended
Apr. 11, 2014
Precedent Agreement \| Transcontinental Gas Pipeline Company LLC [Member]
Purchase Agreements Line Items
Base term of an agreement	25 years
Agreement fixed charge for the base term	$ 37,700,000

Represents the amount of agreement fixed charges for the base term per year.

No definition available.

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