Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Effective Income Tax Rate Reconciliation [Line Items]
Income Tax Expense (Benefit)	$ 3,466	$ 3,382	$ 17,262	$ 18,586
Northwestern Energy Group Inc.
Effective Income Tax Rate Reconciliation [Line Items]
Income Tax Expense (Benefit)	$ 3,500	$ 3,400	$ 17,300	$ 18,600
Effective Income Tax Rate Reconciliation, Percent	12.20%	13.70%	16.30%	15.90%

Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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+ Details