XML 87 R71.htm IDEA: XBRL DOCUMENT
v3.6.0.2

Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Regulatory assets and liabilities |  |  |
Regulatory assets: |  | $ 307,320 | $ 263,258
Regulatory liabilities: |  | 241,852 | 243,118
Net regulatory position |  | 65,468 | 20,140
Regulatory assets not earning a rate of return |  | 255,400 | 224,700
Plant removal and decommissioning costs [Member} |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 176,972 | 182,981
Taxes refundable to customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 11,010 | 17,060
Pension and postretirement benefits [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1] | 9,099 | 4,764
Natural gas costs refundable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [2] | 25,580 | 20,884
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Regulatory liabilities: | [1],[2] | $ 19,191 | 17,429
Pension and postretirement benefits [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [3] | (e) |
Regulatory assets: | [4] | $ 176,025 | 185,832
Taxes recoverable from customers [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Over plant lives |
Regulatory assets: | [4] | $ 28,278 | 27,682
Manufactured gas plant sites remediation [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | 0 |
Regulatory assets: | [4] | $ 18,259 | 18,617
Asset Retirement Obligation Costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | 0 |
Regulatory assets: | [4] | $ 42,580 | 8,000
Natural gas costs recoverable through rate adjustments [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 1 year |
Regulatory assets: | [6] | $ 2,242 | 547
Long-term debt refinancing costs [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 21 years |
Regulatory assets: | [4] | $ 6,248 | 7,031
Costs related to identifying generation development [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Up to 10 years |
Regulatory assets: | [4] | $ 3,407 | 3,808
Other regulatory assets/liabilities [Member] |  |  |
Regulatory assets and liabilities |  |  |
Estimated recovery period | [5] | Largely within 1- 4 years |
Regulatory assets: | [4],[6] | $ 30,281 | $ 11,741
[1] | Included in deferred credits and other liabilities - other 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] | Estimated 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. + Details
Name: | mdu_EstimatedRemainingRecoveryPeriodOfRegulatoryAssets
Namespace Prefix: | mdu_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | mdu_ScheduleOfRegulatoryAssetsAndLiabilitiesLineItems
Namespace Prefix: | mdu_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | us-gaap_NetRegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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 + Details
Name: | us-gaap_RegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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 + Details
Name: | us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_RemovalCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=mdu_TaxesRefundableToCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=mdu_LiabilitiesRefundableGasCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_PensionAndOtherPostretirementPlansCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=mdu_TaxesRecoverableFromCustomersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_EnvironmentalRestorationCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_AssetRetirementObligationCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_AssetRecoverableGasCostsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_LossOnReacquiredDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=mdu_CostsRelatedToIdentifyingGenerationDevelopmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryAssetAxis=us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |