Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Outstanding credit facilities

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

Company	Facility		Facility Limit			Amount Outstanding at December 31, 2017			Amount Outstanding at December 31,  2016			Letters of Credit at December 31, 2017			Expiration Date
			(In millions)
MDU Resources Group, Inc.	Commercial paper/Revolving credit agreement	(a)	$	175.0		$	73.8	(b)	$	111.0	(b)	$	—		5/8/19
Cascade Natural Gas Corporation	Revolving credit agreement		$	75.0	(c)	$	17.3		$	—		$	2.2	(d)	4/24/20
Intermountain Gas Company	Revolving credit agreement		$	85.0	(e)	$	40.0		$	20.9		$	—		4/24/20
Centennial Energy Holdings, Inc.	Commercial paper/Revolving credit agreement	(f)	$	500.0		$	14.6	(b)	$	151.0	(b)	$	—		9/23/21

(a) The commercial paper program is supported by a revolving credit agreement with various banks (provisions allow for increased borrowings, at the option of the Company on stated conditions, up to a maximum of $225.0 million). There were no amounts outstanding under the revolving credit agreement.

(b) Amount outstanding under commercial paper program.

(c) Certain provisions allow for increased borrowings, up to a maximum of $100.0 million.

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

(e) Certain provisions allow for increased borrowings, up to a maximum of $110.0 million.

(f) The commercial paper program is 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 $600.0 million). There were no amounts outstanding under the revolving credit agreement.

Long term debt outstanding

Long-term debt outstanding was as follows:

	Weighed Average Interest Rate at December 31, 2017		December 31, 2017		December 31, 2016
			(In thousands)
Senior Notes due on dates ranging from June 19, 2018 to January 15, 2055	4.71	%	$	1,499,916	$	1,437,831
Commercial paper supported by revolving credit agreements	1.72	%	88,350		262,000
Medium-Term Notes due on dates ranging from September 1, 2020 to March 16, 2029	6.68	%	50,000		50,000
Other notes due on dates ranging from July 1, 2019 to November 30, 2038	5.24	%	24,982		24,471
Credit agreements due on April 24, 2020	3.71	%	57,300		21,793
Less unamortized debt issuance costs			5,694		5,832
Less discount			1		104
Total long-term debt			1,714,853		1,790,159
Less current maturities			148,499		43,598
Net long-term debt			$	1,566,354	$	1,746,561

Schedule of debt maturities

Schedule of Debt Maturities Long-term debt maturities, which excludes unamortized debt issuance costs and discount, for the five years and thereafter following December 31, 2017, were as follows:

	2018		2019		2020		2021		2022		Thereafter
	(In thousands)
Long-term debt maturities	$	148,499	$	125,504	$	73,012	$	15,312	$	147,214	$	1,211,007