Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Right-of-Use Assets—Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(268,923)	(267,606)
Total finance lease assets	33,809	35,126
Lease liabilities—Finance leases
Obligations under finance leases	61,335	61,335
Long-term debt and finance leases due within one year	7,541	7,541
Total finance lease liabilities	$ 68,876	$ 68,876
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, net	$ 2,086	$ 2,293
Total operating lease assets	2,086	2,293
Lease liabilities—Operating leases
Capitalization—Other	1,301	1,550
Other current liabilities	798	838
Total operating lease liabilities	$ 2,099	$ 2,388
Operating Lease, Liability, Noncurrent, Statement of Financial Position	Obligation under Hydro Facility Transactions	Obligation under Hydro Facility Transactions
Operating Lease, Liability, Current, Statement of Financial Position	Other current liabilities	Other current liabilities