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Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Dividends declared per common share | $ 0.7350 | $ 0.7150 | $ 0.6950
Company's subsidiaries net assets restricted from being used to transfer funds to the Company | $ 1,600 |  |
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 | $ 322 |  |
MDU Resources Group, Inc. [Member] |  |  |
Shares of common stock reserved for original issuance under the Stock Purchase Plan and K-Plan (in shares) | 13.9 |  |
Centennial [Member] |  |  |
Maximum distributions to the company as a percentage of consolidated net income after taxes, excluding noncash write-downs, for the preceding fiscal year (in hundredths) | 100.00% |  |

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- Definition The maximum distributions to the company by the subsidiary as a percentage of consolidated net income after taxes,excluding noncash write-downs, for the preceding fiscal year, pursuant to a covenant under a credit agreement. + References No definition available. + Details
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