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Regulatory Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 7,860	$ 16,127
Regulatory assets, long-term	95,537	95,715
Regulatory liabilities, current	43,685	25,408
Regulatory liabilities, long -term	282,997	276,547
Deferred Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	131,584	136,022
Plant Removal Obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	125,681	117,030
Fuel Clause Adjustments
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	18,808	11,350
Regulatory liabilities, long -term	$ 0	0
Regulatory liabilities - Remaining Recovery/Refund Period	1 year
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 17,594	6,885
Regulatory liabilities, long -term	$ 0	0
Regulatory liabilities - Remaining Recovery/Refund Period	1 year
North Dakota PTC Refunds
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	$ 0	0
Regulatory liabilities, long -term	17,115	12,011
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	6,138	6,138
Regulatory liabilities, long -term	7,737	11,307
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	1,145	1,035
Regulatory liabilities, long -term	880	177
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	154	154
Regulatory assets, long-term	86,020	86,134
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	4,145	3,719
Regulatory assets, long-term	$ 145	158
Regulatory assets - Remaining Recovery/Refund Period	2 years
Deferred Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 0	0
Regulatory assets, long-term	7,217	6,940
Fuel Clause Adjustments
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	0	7,294
Regulatory assets, long-term	$ 0	0
Regulatory assets - Remaining Recovery/Refund Period	1 year
Derivative Instruments
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 2,116	4,210
Regulatory assets, long-term	$ 686	0
Regulatory assets - Remaining Recovery/Refund Period	2 years
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 1,445	750
Regulatory assets, long-term	$ 1,469	$ 2,483

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Schedule of Regulatory Assets and Liabilities

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Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

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