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Notes to Condensed Financial Statements (Details 5) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items] |  |  |
2018 | $ 148,499,000 |  |
2019 | 125,504,000 |  |
2020 | 73,012,000 |  |
2021 | 15,312,000 |  |
2022 | 147,214,000 |  |
Mature in years after 2022 | 1,211,007,000 |  |
MDU Resources Group, Inc. [Member] |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |
2018 | 100,000,000.0 |  |
2019 | 74,500,000 |  |
2020 | 700,000 |  |
2021 | 700,000 |  |
2022 | 700,000 |  |
Mature in years after 2022 | 538,100,000 |  |
Cash dividends paid to parent company by consolidated subsidiaries | $ 116,100,000 | $ 115,800,000 | $ 110,600,000

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