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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: | $ 269,586,000 us-gaap_RegulatoryAssets |  | $ 269,586,000 us-gaap_RegulatoryAssets |  | $ 179,766,000 us-gaap_RegulatoryAssets |
Regulatory liabilities: | 386,252,000 us-gaap_RegulatoryLiabilities |  | 386,252,000 us-gaap_RegulatoryLiabilities |  | 367,411,000 us-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 | 229,600,000 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided |  | 229,600,000 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided |  | 163,700,000 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided |
Plant removal and decommissioning costs [Member} |  |  |  |  |  |
Regulatory assets and liabilities |  |  |  |  |  |
Regulatory liabilities: | 338,641,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_RemovalCostsMember | [1] | 338,641,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_RemovalCostsMember | [1] | 308,431,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_RemovalCostsMember | [1]
Taxes refundable to customers [Member] |  |  |  |  |  |
Regulatory assets and liabilities |  |  |  |  |  |
Regulatory liabilities: | 17,772,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_TaxesRefundableToCustomersMember | [1] | 17,772,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_TaxesRefundableToCustomersMember | [1] | 20,180,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_TaxesRefundableToCustomersMember | [1]
Natural gas costs refundable through rate adjustments [Member] |  |  |  |  |  |
Regulatory assets and liabilities |  |  |  |  |  |
Regulatory liabilities: | 13,238,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_LiabilitiesRefundableGasCostsMember | [2] | 13,238,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_LiabilitiesRefundableGasCostsMember | [2] | 16,932,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = mdu_LiabilitiesRefundableGasCostsMember | [2]
Other regulatory assets/liabilities [Member] |  |  |  |  |  |
Regulatory assets and liabilities |  |  |  |  |  |
Regulatory liabilities: | 16,601,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | [1],[2] | 16,601,000 us-gaap_RegulatoryLiabilities / us-gaap_RegulatoryLiabilityAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | [1],[2] | 21,868,000 us-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: | 182,565,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember | [4] | 182,565,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_PensionAndOtherPostretirementPlansCostsMember | [4] | 105,123,000 us-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: | 22,910,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = mdu_TaxesRecoverableFromCustomersMember | [4] | 22,910,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = mdu_TaxesRecoverableFromCustomersMember | [4] | 18,266,000 us-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: | 17,548,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember | [4] | 17,548,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_EnvironmentalRestorationCostsMember | [4] | 15,797,000 us-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: | 19,575,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRecoverableGasCostsMember | [6] | 19,575,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_AssetRecoverableGasCostsMember | [6] | 12,060,000 us-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: | 7,864,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_LossOnReacquiredDebtMember | [4] | 7,864,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_LossOnReacquiredDebtMember | [4] | 8,697,000 us-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: | 4,165,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember | [4] | 4,165,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember | [4] | 4,512,000 us-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: | $ 14,959,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | [4],[6] | $ 14,959,000 us-gaap_RegulatoryAssets / us-gaap_RegulatoryAssetAxis = us-gaap_OtherRegulatoryAssetsLiabilitiesMember | [4],[6] | $ 15,311,000 us-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.

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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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