Fair value measurements (Details 3) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Concentration risks, percentage [Abstract]
Percentage in fixed-income and other investments	54.00%	57.00%	58.00%
Percentage investment in common stock of large-cap companies	19.00%	18.00%	20.00%
Percentage investment in common stock of mid-cap companies	10.00%	9.00%	8.00%
Percentage investment in common stock of small-cap companies	9.00%	9.00%	8.00%
Percentage investment in target date investments	6.00%	5.00%	4.00%
Percentage investment in cash and cash equivalents	2.00%	2.00%	2.00%
Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 122,496	$ 120,348	$ 102,836
Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	122,496	120,348	102,836
Money market funds \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,388	8,917	8,470
Money market funds \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	9,388	8,917	8,470
Insurance contracts* \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	101,632	100,104	83,254
Insurance contracts* \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	101,632	100,104	83,254
Mortgage-backed securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,854	9,946	9,927
Mortgage-backed securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	8,854	9,946	9,927
U.S. Treasury securities \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	2,622	1,381	1,185
U.S. Treasury securities \| Fair value, inputs, level 2 \| Fair value, measurements, recurring
Fair value measurements [Line Items]
Assets, fair value disclosure	$ 2,622	$ 1,381	$ 1,185

Concentration Risks, Percentage

No definition available.

Name:	mdu_ConcentrationRisksPercentageAbstract
Namespace Prefix:	mdu_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of insurance contracts in fixed-income and other investments.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInFixedIncomeAndOtherInvestments
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of insurance contracts in common stock of large-cap companies.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInvestmentInCommonStockOfLargeCapCompanies
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of insurance contracts in common stock of mid-cap companies.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInvestmentInCommonStockOfMidCapCompanies
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of insurance contracts in common stock of small-cap companies.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInvestmentInCommonStockOfSmallCapCompanies
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of insurance contracts in cash and cash equivalents.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInvestmentinCashandCashEquivalents
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The percentage of insurance contracts in target date investments.

+ References

No definition available.

+ Details

Name:	mdu_PercentageInvestmentinTargetDateInvestments
Namespace Prefix:	mdu_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_CashSurrenderValueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: