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Common stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Dividends declared per common share | $ 0.7750 | $ 0.7550 | $ 0.7350
Company's subsidiaries net assets restricted from being used to transfer funds to the Company | $ 1,300 |  |
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 | $ 384 |  |
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% |  |

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