Sales to Non-Members (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Sales to Non-Members
Sales to non-members consisting primarily of capacity and energy sales	$ 38,628	$ 82,624	$ 99,842	$ 134,977

Revenue for bundled services from customers who are non members that receive electrical generation services.

No definition available.

Name:	opc_ElectricGenerationRevenueSalesToNonMembers
Namespace Prefix:	opc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration