Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Right-of-Use Assets—Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(264,092)	(262,774)
Total finance lease assets	38,640	39,958
Lease liabilities—Finance leases
Obligations under finance leases	68,876	68,876
Long-term debt and finance leases due within one year	6,773	6,773
Total finance lease liabilities	$ 75,649	$ 75,649
Finance Lease, Liability, Current, Statement of Financial Position	Long-term debt and finance leases due within one year	Long-term debt and finance leases due within one year
Right-of-Use Assets—Operating leases
Electric plant in service	$ 3,038	$ 3,283
Total operating lease assets	3,038	3,283
Lease liabilities—Operating leases
Capitalization—Other	2,099	2,388
Other current liabilities	952	990
Total operating lease liabilities	$ 3,051	$ 3,378
Operating Lease, Liability, Current, Statement of Financial Position	Other current liabilities	Other current liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position	opc:ObligationUnderHydroFacilityTransactions	opc:ObligationUnderHydroFacilityTransactions