Recently Issued or Adopted Accounting Pronouncements (Details)	9 Months Ended
Sep. 30, 2017 item
Recently Issued or Adopted Accounting Pronouncements
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