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Commitments and contingencies - Guarantees (Details) | Sep. 30, 2025 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
Letters of credit | $ 3,200,000
Outstanding letters of credit | 0
Amount of surety bonds outstanding | $ 13,000,000

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- 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

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- Definition Letters of credit. + References No definition available. + Details
Name: | mdu_LettersOfCredit
Namespace Prefix: | mdu_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 15 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-15 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477548/944-605-55-12 + Details
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Namespace Prefix: | us-gaap_
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