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Long-term borrowings (Details 2) - USD ($) $ in Millions | Mar. 21, 2017 | Dec. 31, 2016 | Nov. 21, 2016 | Nov. 09, 2016
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MDU Resources Group, Inc [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Note purchase agreement |  |  |  | $ 100.0 |
Senior notes |  |  |  | $ 40.0 |
Interest rate |  |  |  | 4.15% |
MDU Resources Group, Inc [Member] | Revolving credit facility [Member] | Long-term debt [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% |  |
MDU Energy Capital, LLC [Member] | Master shelf agreement [Member] | Long-term debt [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Ratio of total debt to adjusted total capitalization as specified in debt convenants |  |  | 70.00% |  |
Ratio of subsidiary debt to subsidiary capitalization as specified in debt convenants |  |  | 65.00% |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  |  | 65.00% |  |
Ratio - EBIT to interest expense |  |  | 150.00% |  |
Cascade Natural Gas Corporation [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Facility limit, maximum borrowing capacity | [1] |  | $ 50.0 |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  |  | 65.00% |  |
Intermountain Gas Company [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Senior notes |  |  |  |  | $ 30.0
Interest rate |  |  |  |  | 4.00%
Intermountain Gas Company [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Facility limit, maximum borrowing capacity | [2] |  | $ 65.0 |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  |  | 65.00% |  |
Centennial Energy Holdings, Inc [Member] | Master shelf agreement [Member] | Long-term debt [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] | Long-term debt [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Facility limit, maximum borrowing capacity |  |  | $ 500.0 |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  |  | 65.00% |  |
Facility, decrease borrowing capacity |  |  | $ 150.0 |  |
WBI Energy Transmission, Inc. [Member] | Uncommitted long term private shelf agreement [Member] | Long-term debt [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Ratio of total debt to total capitalization as specified in debt convenants. |  |  | 55.00% |  |
Long-term private shelf agreement |  |  | $ 175.0 |  |
Long-term private shelf agreement issuance capacity |  |  | 200.0 |  |
Notes outstanding |  |  | 100.0 |  |
Remaining capacity under uncommitted private shelf agreement |  |  | $ 100.0 |  |
Subsequent Event [Member] | MDU Resources Group, Inc [Member] |  |  |  |  |
Debt instrument [Line Items] |  |  |  |  |
Senior notes |  | $ 60.0 |  |  |
Weighted average interest rate |  | 3.61% |  |  |
[1] | Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[2] | Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

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