Income taxes: (Details 3) (State jurisdictions, Maximum)	12 Months Ended
Dec. 31, 2012
State jurisdictions \| Maximum
Income taxes
Period during which state impact of any federal changes remains subject to examination by various states	1 year

Represents the period after formal notification to the states during which state impact of any federal changes remains subject to examination by various states.

No definition available.

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