Leases - Balance sheet impact (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Right-of-Use Assets - Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(267,606)	(262,774)
Total finance lease assets	35,126	39,958
Lease liabilities - Finance leases
Obligations under finance leases	61,335	68,876
Long-term debt and finance leases due within one year	7,541	6,773
Total finance lease liabilities	68,876	75,649
Right-of-Use Assets - Operating leases
Electric plant in service	2,293	3,283
Total operating lease assets	2,293	3,283
Lease liabilities - Operating leases
Capitalization - Other	1,550	2,388
Other current liabilities	838	990
Total operating lease liabilities	$ 2,388	$ 3,378
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Right-of-use assets	Right-of-use assets
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List]	In service	In service
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Obligation under Rocky Mountain transactions	Obligation under Rocky Mountain transactions
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other current liabilities	Other current liabilities