XML 85 R68.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Environmental matters (Details 2) - USD ($) | 6 Months Ended |
--- | --- | ---
Jun. 30, 2017 | Jan. 06, 2017
Portland Harbor Site [Member] |  |
Site Contingency [Line Items] |  |
Environmental matters investigative costs | $ 100,000,000 |
Environmental matters, estimated costs |  | $ 1,000,000,000
Eugene, OR Manufactured Gas Plant Site [Member] |  |
Site Contingency [Line Items] |  |
Total estimated costs for site remediation | 3,500,000 |
Incurred costs for site remediation | $ 320,000 |
Percent of ongoing costs paid | 50.00% |
Estimated proportional share of cleanup liability | 50.00% |
Environmental matters accrual for site remediation | $ 1,600,000 |
Bremerton, WA Manufactured Gas Plant Site [Member] |  |
Site Contingency [Line Items] |  |
Environmental matters accrual for site remediation | 6,400,000 |
Total estimated costs for site remedial investigation and feasibility study | 7,600,000 |
Incurred costs for site remedial investigation and feasibility study | 200,000 |
Environmental matters accrual of investigative costs | 7,400,000 |
Bellingham, WA Manufactured Gas Plant Site [Member] |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate | 8,000,000 |
Minimum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member] |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate | 340,000 |
Maximum [Member] | Bremerton, WA Manufactured Gas Plant Site [Member] |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate | $ 6,400,000 |

X
- Definition Environmental matters accrual for remedial investigation and feasibility study costs. + References No definition available. + Details
Name: | mdu_EnvironmentalMattersAccrualForRemedialInvestigationandFeasibilityStudyCosts
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Environmental matters accrual for site remediation + References No definition available. + Details
Name: | mdu_EnvironmentalMattersAccrualForSiteRemediation
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Environmental matters, estimated costs. + References No definition available. + Details
Name: | mdu_EnvironmentalMattersEstimatedCosts
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Estimated investigative costs. + References No definition available. + Details
Name: | mdu_EnvironmentalMattersInvestigativeCosts
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The estimated proportional share of the clean-up costs. + References No definition available. + Details
Name: | mdu_EstimatedProportionalShareOfCleanupLiability
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Incurred costs for site remedial investigation and feasibility study + References No definition available. + Details
Name: | mdu_IncurredCostsForSiteRemedialInvestigationAndFeasibilityStudy
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Incurred costs for site remediation + References No definition available. + Details
Name: | mdu_IncurredCostsForSiteRemediation
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percent of ongoing costs paid. + References No definition available. + Details
Name: | mdu_PercentOfOngoingCostsPaid
Namespace Prefix: | mdu_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Total estimated costs for site remedial investigation and feasibility study + References No definition available. + Details
Name: | mdu_TotalEstimatedCostsForSiteRemedialInvestigationAndFeasibilityStudy
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Total estimated costs for site remediation + References No definition available. + Details
Name: | mdu_TotalEstimatedCostsForSiteRemediation
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

X
- Definition Best estimate of the loss exposure for reasonably possible environmental contingencies at an individual site for which no accrual has been recorded. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (c),(d)(2) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 + Details
Name: | us-gaap_SiteContingencyLossExposureNotAccruedBestEstimate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_EnvironmentalRemediationSiteAxis=mdu_PortlandHarborSiteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnvironmentalRemediationSiteAxis=mdu_EugeneORManufacturedGasPlantSiteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnvironmentalRemediationSiteAxis=mdu_BremertonWAManufacturedGasPlantSiteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EnvironmentalRemediationSiteAxis=mdu_BellinghamWAManufacturedGasPlantSiteMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |