Leases - Balance Sheet Impact (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Right of Use Assets
Right-of-Use Assets, Finance	$ 49,181,000	$ 50,499,000
Right-of-Use Assets, Operating	$ 5,714,000
Right of Use Assets, Operating, Statement of Financial Position	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentPlantInService	us-gaap:PublicUtilitiesPropertyPlantAndEquipmentPlantInService
Total leased assets	$ 54,895,000	$ 50,499,000
Finance lease, right-of-use assets, accumulated depreciation	253,551,000	252,233,000
Liabilities, Current
Finance	$ 5,462,000	$ 5,462,000
Finance Lease, Liability, Current, Statement of Financial Position	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent	us-gaap:LongTermDebtAndCapitalLeaseObligationsCurrent
Operating	$ 3,533,000
Operating Lease, Liability, Current, Statement of Financial Position	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Liabilities, Noncurrent
Finance	$ 81,730,000	$ 81,730,000
Finance Lease, Liability, Noncurrent, Statement of Financial Position	us-gaap:FinanceLeaseLiabilityNoncurrent	us-gaap:FinanceLeaseLiabilityNoncurrent
Operating	$ 2,751,000
Operating Lease, Liability, Noncurrent, Statement of Financial Position	opc:ObligationUnderHydroFacilityTransactions	opc:ObligationUnderHydroFacilityTransactions
Total leased liabilities	$ 93,476,000	$ 87,192,000