Debt (Tables)

3 Months Ended

Mar. 31, 2026

Debt Disclosure [Abstract]

Schedule of Line of Credit Facilities

The following table summarizes the outstanding revolving credit facilities of the Company and its subsidiaries:

Company	Debt-to-Total Capitalization Ratio at March 31, 2026		Provisions for Increased Borrowings, up to a maximum of:		Facility Limit		Amount Outstanding at March 31, 2026		Amount Outstanding at March 31, 2025		Amount Outstanding at December 31, 2025		Letters of Credit at March 31, 2026		Expiration Date
					(In millions)
Montana-Dakota Utilities Co.	51	%	$	250.0	$	200.0	$	104.6	$	51.1	$	132.0	$	—	12/11/30
Cascade Natural Gas Corporation	50	%	$	225.0	$	175.0	$	124.3	$	21.4	$	96.5	$	2.2	12/11/30
Intermountain Gas Company	51	%	$	225.0	$	175.0	$	59.8	$	83.5	$	67.3	$	—	12/11/30
MDU Resources Group, Inc.	47	%	$	250.0	$	200.0	$	43.5	$	—	$	32.9	$	1.0	12/11/30

Schedule of Long-Term Debt Outstanding

Long-term debt outstanding was as follows:

	Weighted Average Interest Rate at March 31, 2026		March 31, 2026		March 31, 2025		December 31, 2025
			(In thousands)
Senior Notes due on dates ranging from July 15, 2026 to June 15, 2062	4.83	%	$	2,110,000	$	1,947,000	$	2,010,000
Credit agreements due on December 11, 2030	5.47	%	227,600		104,900		196,700
Term Loan Agreements due on dates ranging from January 31, 2027 to April 1, 2039	4.04	%	126,900		61,600		310,900
Commercial paper supported by revolving credit agreement	4.16	%	104,617		51,100		132,000
Medium-Term Notes due on dates ranging from September 15, 2027 to March 16, 2029	7.32	%	35,000		35,000		35,000
Other notes due on November 30, 2038	6.00	%	324		342		329
Less unamortized debt issuance costs			8,445		6,266		8,074
Total long-term debt			2,595,996		2,193,676		2,676,855
Less current maturities			214,700		161,700		144,700
Net long-term debt			$	2,381,296	$	2,031,976	$	2,532,155