General (Details)	3 Months Ended
Mar. 31, 2018 item
General
Number of electric distribution cooperative members	38
Margin policy
Margins for interest ratio	1.10

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