General (Details)	9 Months Ended
Sep. 30, 2017 item
General
Number of electric distribution cooperative members	38

Represents the number of electric distribution cooperative members.

No definition available.

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