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Regulatory Matters (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 9,433	$ 9,962
Regulatory assets, long-term	97,936	98,673
Regulatory liabilities, current	21,753	29,307
Regulatory liabilities, long -term	296,216	288,928
Deferred Income Taxes
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	124,832	130,387
Plant Removal Obligations
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	$ 132,438	126,263
Fuel Clause Adjustments
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities - Remaining Recovery/Refund Period	1 year
Regulatory liabilities, current	$ 5,197	11,432
Regulatory liabilities, long -term	$ 0	0
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities - Remaining Recovery/Refund Period	1 year
Regulatory liabilities, current	$ 10,888	14,255
Regulatory liabilities, long -term	0	0
North Dakota PTC Refunds
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	0	0
Regulatory liabilities, long -term	26,348	20,099
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	2,547	2,547
Regulatory liabilities, long -term	10,350	10,758
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory liabilities, current	3,121	1,073
Regulatory liabilities, long -term	2,248	1,421
Pension and Other Postretirement Benefit Plans
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	0	0
Regulatory assets, long-term	87,182	88,161
Alternative Revenue Program Riders
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	4,610	4,257
Regulatory assets, long-term	$ 146	195
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	9,070	8,944
Fuel Clause Adjustments
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	3,229	2,218
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	$ 795	1,989
Regulatory assets, long-term	$ 512	0
Regulatory assets - Remaining Recovery/Refund Period	2 years
Other
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory assets - current	$ 799	1,498
Regulatory assets, long-term	$ 1,026	$ 1,373

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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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