Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Right-of-use assets—Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(279,465)	(278,586)
Total finance lease assets	$ 23,267	$ 24,146
Finance lease, right-of-use asset, statement of financial position	Right-of-use assets—finance leases	Right-of-use assets—finance leases
Lease liabilities—Finance leases
Obligations under finance leases	$ 43,586	$ 43,586
Long-term debt and finance leases due within one year	9,351	9,351
Total finance lease liabilities	$ 52,937	$ 52,937
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	$ 6,181	$ 6,587
Total operating lease assets	$ 6,181	$ 6,587
Operating lease, right-of-use asset, statement of financial position	In service	In service
Lease liabilities—Operating leases
Capitalization—Other	$ 4,786	$ 5,152
Other current liabilities	1,395	1,529
Total operating lease liabilities	$ 6,181	$ 6,681
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