Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Regulatory Assets [Line Items]
Current regulatory assets	$ 1,358	$ 1,412
Regulatory assets, noncurrent	18,312	20,022
Total regulatory assets	1,358	21,434
Regulatory Liabilities [Line Items]
Regulatory liabilities	388,433	355,725
Total regulatory liabilities	388,465	358,957
Right-of-use assets	10,238	10,692
Fuel recovery
Regulatory Liabilities [Line Items]
Current regulatory liabilities	32	3,232
Postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory liabilities	40,980	39,391
Deferred federal taxes-liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	206,527	206,527
Deferred state taxes - liability
Regulatory Liabilities [Line Items]
Regulatory liabilities	12,120	12,120
Customer tax refund [Domain]
Regulatory Liabilities [Line Items]
Regulatory liabilities	102,059	75,694
Asset retirement obligations, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	26,747	21,993
Accrued Interest on Deferred Regulatory Tax Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	7,700
Regulatory charges resulting from tax rate changes [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	23,600
Levelized depreciation
Regulatory Assets [Line Items]
Current regulatory assets	1,251	1,360
Regulatory assets, noncurrent	13,581	14,908
Fuel recovery
Regulatory Assets [Line Items]
Current regulatory assets	107	52
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets, noncurrent	$ 4,731	$ 5,114