Estimated discounted future cash flows of total proved reserves (Details 6) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Future cash inflows	$ 5,185,500	$ 4,507,000	$ 3,696,200
Future production costs	1,856,900	1,734,800	1,536,500
Future development costs	570,200	403,000	301,600
Future net cash flows before income taxes	2,758,400	2,369,200	1,858,100
Future income tax expense	686,100	545,200	304,900
Future net cash flows	2,072,300	1,824,000	1,553,200
10% annual discount for estimated timing of cash flows	997,400	810,000	669,800
Discounted future net cash flows relating to proved oil, NGL and natural gas reserves	$ 1,074,900	$ 1,014,000	$ 883,400	$ 978,800

Discounted future net cash flows before income taxes

No definition available.

Name:	mdu_DiscountedFutureNetCashFlowsBeforeIncomeTaxes
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

+ References

+ Details

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

- Definition

Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and gas reserves.

+ References

+ Details

Name:	us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesDevelopmentCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and gas reserves.

+ References

+ Details

Name:	us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and gas reserves.

+ References

+ Details

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

- Definition

Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and gas reserves.

+ References

+ Details

Name:	us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year.

+ References

+ Details

Name:	us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

Amount of discounted future accumulated cash inflow relating to proved oil and gas reserves.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 30

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 35

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62500-109447

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=34713648&loc=d3e63123-109448

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 31

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447

+ Details

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