Debt (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Schedule of Line of Credit Facilities

The following table summarizes the outstanding revolving credit facilities of the Company's subsidiaries:

Company	Facility	Facility Limit			Amount Outstanding at December 31, 2023		Amount Outstanding at December 31, 2022		Letters of Credit at December 31, 2023			Expiration Date
		(In millions)
Montana-Dakota Utilities Co.	Commercial paper/Revolving credit agreement (a)	$	200.0		$	144.2	$	117.5	$	—		10/18/28
Cascade Natural Gas Corporation	Revolving credit agreement	$	100.0	(b)	$	15.4	$	44.4	$	25.0	(c)	11/30/27
Intermountain Gas Company	Revolving credit agreement	$	100.0	(d)	$	30.7	$	85.6	$	—		10/13/27
Centennial Energy Holdings, Inc.	Commercial paper/Revolving credit agreement (e)	$	—		$	—	$	231.6	$	—		12/19/24
MDU Resources Group, Inc.	Revolving credit agreement	$	150.0		$	—	$	—	$	—		5/29/24
MDU Resources Group, Inc.	Revolving credit agreement	$	200.0	(f)	$	—	$	—	$	8.9		5/31/28

(a)The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Montana-Dakota on stated conditions, up to a maximum of $250.0 million). At December 31, 2023 and 2022, there were no amounts outstanding under the revolving credit agreement.

(b)Certain provisions allow for increased borrowings, up to a maximum of $125.0 million.

(c)Outstanding letter(s) of credit reduce the amount available under the credit agreement.

(d)Certain provisions allow for increased borrowings, up to a maximum of $125.0 million.

(e)Centennial repaid all of its outstanding debt in the second quarter of 2023, which was funded by the Knife River repayment and the Company entering into various new debt instruments. The commercial paper program was supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of Centennial on stated conditions, up to a maximum of $700.0 million). At December 31, 2022, there was no amount outstanding under the revolving credit agreement.

(f)Certain provisions allow for increased borrowings, up to a maximum of $250.0 million.

Schedule of Long-Term Debt Instruments

Long-term debt outstanding was as follows:

	Weighted Average Interest Rate at December 31, 2023		2023		2022
			(In thousands)
Senior Notes due on dates ranging from July 15, 2024 to June 15, 2062	4.46	%	$	1,882,000	$	1,848,500
Commercial paper supported by revolving credit agreements	5.94	%	144,200		349,050
Term Loan Agreements due on May 31, 2025 and September 3, 2032	6.51	%	196,300		7,000
Credit agreements due on October 13, 2027 and November 30, 2027	8.50	%	46,100		130,000
Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029	7.32	%	35,000		35,000
Other notes due on dates ranging from May 31, 2028 to November 30, 2038	2.21	%	980		1,614
Less unamortized debt issuance costs			6,357		5,211
Less discount			—		286
Total long-term debt			2,298,223		2,365,667
Less current maturities			61,319		47,819
Net long-term debt			$	2,236,904	$	2,317,848

Schedule of Maturities of Long-Term Debt

Long-term debt maturities, which excludes unamortized debt issuance costs and discount, for the five years and thereafter following December 31, 2023, were as follows:

	2024		2025		2026		2027		2028		Thereafter
	(In thousands)
Long-term debt maturities	$	61,319	$	347,700	$	140,700	$	66,800	$	219,900	$	1,468,161