Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Right-of-use assets - Finance leases
Right-of-Use Assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(256,186)	(252,233)
Total finance lease assets	46,546	50,499
Lease liabilities - Finance leases
Obligations under finance leases	78,771	81,730
Long-term debt and finance leases due within one year	$ 5,763	$ 5,462
Finance Lease, Liability, Current, Statement of Financial Position	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Total finance lease liabilities	$ 84,534	$ 87,192
Right-of-Use Assets - Operating leases
Electric plant in service	$ 4,068
Right of Use Assets, Operating, Statement of Financial Position	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentPlantInService	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentPlantInService
Total operating lease assets	$ 4,068
Lease liabilities - Operating leases
Capitalization - Other	$ 2,448
Operating Lease, Liability, Noncurrent, Statement of Financial Position	opc:ObligationUnderHydroFacilityTransactions	opc:ObligationUnderHydroFacilityTransactions
Other current liabilities	$ 2,115
Operating Lease, Liability, Current, Statement of Financial Position	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Total operating lease liabilities	$ 4,563