Common stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends declared per common share	$ 0.7950	$ 0.7750	$ 0.7550
Company's subsidiaries net assets restricted from being used to transfer funds to the Company	$ 1,100
Credit agreement limitation on company ratio of funded debt to capitalization (excluding subsidiaries)	65.00%
Company's (excluding its subsidiaries) net assets restricted from use for dividend payments	$ 424
MDU Resources Group, Inc. [Member]
Shares of common stock reserved for original issuance under the K-Plan (in shares)	7.8
Centennial [Member]
Maximum distributions to the company as a ratio of average consolidated indebtedness to consolidated EBITDA	300.00%

Company's (excluding its subsidiaries) net assets restricted from use for dividend payments.

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- Definition

The maximum distributions to the company by the subsidiary as a ratio of average consolidated indebtedness, as of the last day of fiscal quarter and the three previous quarters, to consolidated EBITDA.

+ References

No definition available.

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- Definition

Ratio of earnings before interest and taxes to interest expense as specified in debt convenants.

+ References

No definition available.

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- Definition

Amount of restricted net assets of consolidated and unconsolidated subsidiaries as of the end of the most recently completed fiscal year.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e)(3)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

Aggregate number of common shares reserved for future issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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