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Regulatory assets and liabilities (Details) (USD $)

0 Months Ended

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Regulatory assets and liabilities

Regulatory assets:

$ 269,586,000us-gaap_RegulatoryAssets

$ 269,586,000us-gaap_RegulatoryAssets

$ 179,766,000us-gaap_RegulatoryAssets

Regulatory liabilities:

386,252,000us-gaap_RegulatoryLiabilities

386,252,000us-gaap_RegulatoryLiabilities

367,411,000us-gaap_RegulatoryLiabilities

Net regulatory position

(116,666,000)us-gaap_NetRegulatoryAssets

(116,666,000)us-gaap_NetRegulatoryAssets

(187,645,000)us-gaap_NetRegulatoryAssets

Regulatory assets not earning a rate of return

229,600,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided

229,600,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided

163,700,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided

Plant removal and decommissioning costs [Member}

Regulatory assets and liabilities

Regulatory liabilities:

338,641,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember

^[1]

338,641,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember

^[1]

308,431,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember

^[1]

Taxes refundable to customers [Member]

Regulatory assets and liabilities

Regulatory liabilities:

17,772,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_TaxesRefundableToCustomersMember

^[1]

17,772,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_TaxesRefundableToCustomersMember

^[1]

20,180,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_TaxesRefundableToCustomersMember

^[1]

Natural gas costs refundable through rate adjustments [Member]

Regulatory assets and liabilities

Regulatory liabilities:

13,238,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_LiabilitiesRefundableGasCostsMember

^[2]

13,238,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_LiabilitiesRefundableGasCostsMember

^[2]

16,932,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= mdu_LiabilitiesRefundableGasCostsMember

^[2]

Other regulatory assets/liabilities [Member]

Regulatory assets and liabilities

Regulatory liabilities:

16,601,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[1],[2]

16,601,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[1],[2]

21,868,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[1],[2]

Pension and postretirement benefits [Member]

Regulatory assets and liabilities

Estimated recovery period

(e)

^[3]

Regulatory assets:

182,565,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember

^[4]

182,565,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember

^[4]

105,123,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember

^[4]

Taxes recoverable from customers [Member]

Regulatory assets and liabilities

Estimated recovery period

Over plant lives

^[5]

Regulatory assets:

22,910,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_TaxesRecoverableFromCustomersMember

^[4]

22,910,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_TaxesRecoverableFromCustomersMember

^[4]

18,266,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_TaxesRecoverableFromCustomersMember

^[4]

Manufactured gas plant sites remediation [Member]

Regulatory assets and liabilities

Estimated recovery period

Up to 3 years

^[5]

Regulatory assets:

17,548,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_EnvironmentalRestorationCostsMember

^[4]

17,548,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_EnvironmentalRestorationCostsMember

^[4]

15,797,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_EnvironmentalRestorationCostsMember

^[4]

Natural gas costs recoverable through rate adjustments [Member]

Regulatory assets and liabilities

Estimated recovery period

Up to 28 months

^[5]

Regulatory assets:

19,575,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRecoverableGasCostsMember

^[6]

19,575,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRecoverableGasCostsMember

^[6]

12,060,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_AssetRecoverableGasCostsMember

^[6]

Long-term debt refinancing costs [Member]

Regulatory assets and liabilities

Estimated recovery period

Up to 23 years

^[5]

Regulatory assets:

7,864,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember

^[4]

7,864,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember

^[4]

8,697,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember

^[4]

Costs related to identifying generation development [Member]

Regulatory assets and liabilities

Estimated recovery period

Up to 12 years

^[5]

Regulatory assets:

4,165,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember

^[4]

4,165,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember

^[4]

4,512,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember

^[4]

Other regulatory assets/liabilities [Member]

Regulatory assets and liabilities

Estimated recovery period

Largely within 1- 4 years

^[5]

Regulatory assets:

$ 14,959,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[4],[6]

$ 14,959,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[4],[6]

$ 15,311,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember

^[4],[6]

[1]	Included in other liabilities on the Consolidated Balance Sheets.
[2]	Included in other accrued liabilities on the Consolidated Balance Sheets.
[3]	Recovered as expense is incurred or cash contributions are made.
[4]	Included in deferred charges and other assets - other on the Consolidated Balance Sheets.
[5]	stimated recovery period for regulatory assets currently being recovered in rates charged to customers.
[6]	Included in prepayments and other current assets on the Consolidated Balance Sheets.

X

- Definition

Estimated recovery period of regulatory assets.

+ References

No definition available.

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- Definition

The net amount of all regulatory assets less all regulatory liabilities as of the end of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

+ Details

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X

- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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X

- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 405

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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- Definition

Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 55

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6591318&loc=d3e44360-110383

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