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Inventories and natural gas in storage (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Inventory Disclosure [Abstract]
Aggregates held for resale	$ 112,029,000mdu_AggregatesHeldForResaleInventory	$ 108,161,000mdu_AggregatesHeldForResaleInventory	$ 104,106,000mdu_AggregatesHeldForResaleInventory
Asphalt oil	89,578,000mdu_AsphaltOilInventory	42,135,000mdu_AsphaltOilInventory	66,292,000mdu_AsphaltOilInventory
Materials and supplies	65,599,000us-gaap_InventoryRawMaterialsAndSupplies	65,683,000us-gaap_InventoryRawMaterialsAndSupplies	68,809,000us-gaap_InventoryRawMaterialsAndSupplies
Merchandise for resale	15,688,000us-gaap_InventoryFinishedGoods	24,420,000us-gaap_InventoryFinishedGoods	22,463,000us-gaap_InventoryFinishedGoods
Natural gas in storage (current)	9,303,000us-gaap_EnergyRelatedInventoryNaturalGasInStorage	19,302,000us-gaap_EnergyRelatedInventoryNaturalGasInStorage	6,129,000us-gaap_EnergyRelatedInventoryNaturalGasInStorage
Other	44,401,000us-gaap_OtherInventoryNetOfReserves	41,110,000us-gaap_OtherInventoryNetOfReserves	33,533,000us-gaap_OtherInventoryNetOfReserves
Total	336,598,000us-gaap_InventoryNet	300,811,000us-gaap_InventoryNet	301,332,000us-gaap_InventoryNet
Natural gas in storage noncurrent	$ 49,300,000mdu_NaturalGasInStorageNoncurrent	$ 49,300,000mdu_NaturalGasInStorageNoncurrent	$ 47,400,000mdu_NaturalGasInStorageNoncurrent

X

- Definition

Aggregated amount of aggregates held for resale. This amount is net of valuation reserves and adjustments.

+ References

No definition available.

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- Definition

Aggregated amount of asphalt oil inventory. This amount is net of valuation reserves and adjustments.

+ References

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- Definition

Carrying amount as of the balance sheet date of natural gas in storage, which largely represents the cost of gas required to maintain pressure levels for normal operating purposes, and deemed to be noncurrent asset.

+ References

No definition available.

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- Definition

Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 6

-Subparagraph a

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- Definition

Amount before valuation and LIFO reserves of completed merchandise or goods expected to be sold within one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 6

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- Definition

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Gross amount of unprocessed materials to be used in manufacturing or production process and supplies that will be consumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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Carrying amount, net of valuation reserves and allowances, as of the balance sheet date of inventories not otherwise specified in the taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

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