Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Right-of-Use Assets—Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(266,288)	(262,774)
Total finance lease assets	36,444	39,958
Lease liabilities—Finance leases
Obligations under finance leases	65,207	68,876
Long-term debt and finance leases due within one year	7,146	6,773
Total finance lease liabilities	$ 72,353	$ 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	$ 2,540	$ 3,283
Total operating lease assets	2,540	3,283
Lease liabilities—Operating leases
Capitalization—Other	1,677	2,388
Other current liabilities	876	990
Total operating lease liabilities	$ 2,553	$ 3,378
Operating Lease, Liability, Current, Statement of Financial Position	Other current liabilities	Other current liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position	Obligation under Hydro Facility Transactions	Obligation under Hydro Facility Transactions