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Long-term borrowings (Details 2) - Long-term debt [Member] - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 22, 2017
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MDU Resources Group, Inc [Member] | Revolving credit facility [Member] |  |  |
Debt instrument [Line Items] |  |  |
Ratio of funded debt to total capitalization as specified in debt convenants |  | 65.00% |
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants |  | 65.00% |
Cascade Natural Gas Corporation [Member] | Revolving credit facility [Member] |  |  |
Debt instrument [Line Items] |  |  |
Facility limit, amended borrowing capacity |  | $ 50.0 |
Facility limit, maximum borrowing capacity | [1] | $ 75.0 |
Ratio of total debt to total capitalization as specified in debt convenants. |  | 65.00% |
Intermountain Gas Company [Member] | Revolving credit facility [Member] |  |  |
Debt instrument [Line Items] |  |  |
Facility limit, amended borrowing capacity |  | $ 65.0 |
Facility limit, maximum borrowing capacity | [2] | $ 85.0 |
Ratio of total debt to total capitalization as specified in debt convenants. |  | 65.00% |
Centennial Energy Holdings, Inc [Member] | Master shelf agreement [Member] |  |  |
Debt instrument [Line Items] |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  | 60.00% |
Ratio of EBITDA to interest expense |  | 175.00% |
Centennial Energy Holdings, Inc [Member] | Revolving credit facility [Member] |  |  |
Debt instrument [Line Items] |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  | 65.00% |
WBI Energy Transmission, Inc. [Member] | Uncommitted long term private shelf agreement [Member] |  |  |
Debt instrument [Line Items] |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  | 55.00% |
Long-term private shelf agreement issuance capacity |  | $ 200.0 |
Notes outstanding |  | 100.0 |
Remaining capacity under uncommitted private shelf agreement |  | $ 100.0 |
Contracted to issue additional debt |  |  | $ 40.0
Weighted average interest rate |  |  | 4.18%
[1] | Certain provisions allow for increased borrowings, up to a maximum of $100.0 million.
[2] | Certain provisions allow for increased borrowings, up to a maximum of $110.0 million.

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