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Long-term borrowings (Details 3) $ in Millions | Dec. 31, 2015 USD ($)
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Debt instrument [Line Items] |  |
Letters of credit | $ 57.3 |
MDU Resources Group, Inc [Member] | Commercial paper revolving credit agreement [Member] | Long-term debt [Member] |  |
Debt instrument [Line Items] |  |
Facility limit, maximum borrowing capacity | $ 175.0 |
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 Resources Group, Inc [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |
Debt instrument [Line Items] |  |
Letters of credit | $ 0.0 |
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 | $ 50.0 | [1]
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Letters of credit | $ 2.2 | [2]
Intermountain Gas Company [Member] | Revolving credit facility [Member] | Long-term debt [Member] |  |
Debt instrument [Line Items] |  |
Facility limit, maximum borrowing capacity | $ 65.0 | [3]
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Letters of credit | $ 0.0 |
Centennial Energy Holdings, Inc [Member] | Commercial paper revolving credit agreement [Member] | Long-term debt [Member] |  |
Debt instrument [Line Items] |  |
Facility limit, maximum borrowing capacity | $ 650.0 |
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] |  |
Ratio of total debt to total capitalization as specified in debt convenants. | 65.00% |
Letters of credit | $ 39.4 |
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 |
Notes outstanding | $ 100.0 |
[1] | Certain provisions allow for increased borrowings, up to a maximum of $75.0 million.
[2] | Outstanding letter(s) of credit reduce the amount available under the credit agreement.
[3] | Certain provisions allow for increased borrowings, up to a maximum of $90.0 million.

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