Debt - Schedule of Credit Facilities (Details) - USD ($)	Dec. 31, 2024	Dec. 31, 2023
Line of Credit Facility [Line Items]
Letters of credit at end of period	$ 0
Credit Agreement Expiring October 2028 | Revolving Credit Facility | Montana-Dakota Utilities Co.
Line of Credit Facility [Line Items]
Facility Limit	200,000,000.0
Amount outstanding, end of period	81,400,000	$ 144,200,000
Letters of credit at end of period	0	0
Option to increase borrowings, maximum amount	250,000,000
Credit Agreement Expiring June 2029 | Revolving Credit Facility | Cascade Natural Gas Corporation
Line of Credit Facility [Line Items]
Facility Limit	175,000,000.0
Amount outstanding, end of period	64,600,000	15,400,000
Letters of credit at end of period	2,200,000
Option to increase borrowings, maximum amount	225,000,000
Credit Agreement Expiring June 2029 | Revolving Credit Facility | Intermountain Gas Company
Line of Credit Facility [Line Items]
Facility Limit	175,000,000.0
Amount outstanding, end of period	105,100,000	30,700,000
Letters of credit at end of period	0
Credit Agreement Expiring May 2028 | Revolving Credit Facility | MDU Resources Group, Inc.
Line of Credit Facility [Line Items]
Facility Limit	200,000,000.0
Amount outstanding, end of period	0	$ 0
Letters of credit at end of period	12,100,000
Option to increase borrowings, maximum amount	$ 250,000,000