Long-term borrowings (Details 2) - Long-term debt $ in Millions	Dec. 31, 2018 USD ($)
MDU Resources Group, Inc. \| Revolving credit facility
Debt instrument [Line Items]
Ratio of funded debt to total capitalization as specified in debt covenants	65.00%
Ratio of funded debt to capitalization - Company alone, as specified in debt convenants	65.00%
Ratio of total debt to total capitalization	45.00%
Cascade Natural Gas Corporation \| Revolving credit facility
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Ratio of total debt to total capitalization	51.00%
Intermountain Gas Company \| Revolving credit facility
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Ratio of total debt to total capitalization	49.00%
Centennial Energy Holdings, Inc. \| Revolving credit facility
Debt instrument [Line Items]
Ratio of total debt to total capitalization as specified in debt covenants	65.00%
Ratio of total debt to total capitalization	38.00%
WBI Energy Transmission, Inc. \| Uncommitted long term private shelf agreement
Debt instrument [Line Items]
Long-term private shelf agreement issuance capacity	$ 200.0
Notes outstanding	140.0
Remaining capacity under uncommitted private shelf agreement	$ 60.0
Ratio of total debt to total capitalization as specified in debt covenants	55.00%
Ratio of total debt to total capitalization	40.00%

Long-term private shelf agreement amended issuance capacity.

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- Definition

Ratio of funded debt to total capitalization as specified in debt convenants.

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- Definition

Ratio of total debt to total capitalization as specified in debt convenants.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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