Debt - Credit Facilities (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
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	144,200,000	$ 117,500,000
Letters of credit at end of period	0	0
Option to increase borrowings, maximum amount	250,000,000
Credit Agreement Expiring November 2027 | Revolving Credit Facility | Cascade Natural Gas Corporation
Line of Credit Facility [Line Items]
Facility Limit	100,000,000.0
Amount outstanding, end of period	15,400,000	44,400,000
Letters of credit at end of period	25,000,000.0
Option to increase borrowings, maximum amount	125,000,000
Credit Agreement Expiring October 2027 | Revolving Credit Facility | Intermountain Gas Company
Line of Credit Facility [Line Items]
Facility Limit	100,000,000.0
Amount outstanding, end of period	30,700,000	85,600,000
Letters of credit at end of period	0
Option to increase borrowings, maximum amount	125,000,000
Credit Agreement Expiring December 2024 | Revolving Credit Facility | Centennial Energy Holdings, Inc.
Line of Credit Facility [Line Items]
Facility Limit	0
Amount outstanding, end of period	0	231,600,000
Letters of credit at end of period	0	0
Option to increase borrowings, maximum amount	700,000,000
Credit Agreement Expiring May 2024 | Revolving Credit Facility | MDU Resources Group, Inc.
Line of Credit Facility [Line Items]
Facility Limit	150,000,000.0
Amount outstanding, end of period	0	0
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	8,900,000
Option to increase borrowings, maximum amount	$ 250,000,000