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Guarantees (Details 3) | Mar. 31, 2018 USD ($)
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Guarantor Obligations [Line Items] |
Guarantor obligations, maximum exposure, undiscounted | $ 134,800,000
Fixed maximum amounts guaranteed by year 2018 | 2,700,000
Fixed maximum amounts guaranteed by year 2019 | 63,000,000.0
Fixed maximum amounts guaranteed by year 2020 | 62,100,000
Fixed maximum amounts guaranteed by year 2021 | 500,000
Fixed maximum amounts guaranteed by year 2022 | 500,000
Fixed maximum amounts guaranteed, thereafter | 2,000,000.0
No scheduled maturity date | 4,000,000.0
Amount outstanding under guarantees that is reflected on balance sheet | 0
Letters of credit | 34,000,000.0
Letters of credit set to expire - 2018 | 33,300,000
Letters of credit set to expire - 2019 | 700,000
Outstanding letters of credit | 0
Amount of surety bonds outstanding | 738,700,000
Financial guarantee [Member] |
Guarantor Obligations [Line Items] |
Guarantor obligations, maximum exposure, undiscounted | $ 55,100,000

X
- Definition Amount of surety bonds outstanding related to construction contracts and reclamation obligations. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount outstanding under gurantees that is reflected on the balance sheet. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed by year five. + References No definition available. + Details
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Namespace Prefix: | mdu_
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Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed by year four. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed with no maturity date. + References No definition available. + Details
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Namespace Prefix: | mdu_
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X
- Definition Fixed maximum amounts guaranteed by year one. + References No definition available. + Details
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Balance Type: | credit
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X
- Definition Fixed maximum amounts guaranteed by year three. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed by year two. + References No definition available. + Details
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Namespace Prefix: | mdu_
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Balance Type: | credit
Period Type: | instant

X
- Definition Fixed maximum amounts guaranteed, thereafter. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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X
- Definition Letters of credit. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Letters of credit set to expire current year. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Letters of credit set to expire in next fiscal year. + References No definition available. + Details
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Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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
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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
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- Details
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