Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Current regulatory assets	$ 1,412	$ 978
Regulatory assets, noncurrent	20,022	21,935
Total regulatory assets	21,434	22,913
Regulatory Liabilities [Line Items]
Regulatory liabilities	355,725	323,032
Total regulatory liabilities	358,957	327,153
Fuel recovery
Regulatory Liabilities [Line Items]
Current regulatory liabilities	3,232	4,121
Postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory liabilities	39,391	37,722
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	75,694	49,598
Asset retirement obligations, net [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21,993	17,065
Accrued Interest on Deferred Regulatory Tax Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	5,000
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	1,360	978
Regulatory assets, noncurrent	14,908	16,437
Fuel recovery
Regulatory Assets [Line Items]
Current regulatory assets	52	0
Grossed-up deferred taxes on equity funds used during construction
Regulatory Assets [Line Items]
Regulatory assets, noncurrent	$ 5,114	$ 5,498