Proved reserves (Details 5) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 MBoe	Dec. 31, 2013 MBoe	Dec. 31, 2012 MBoe	Dec. 31, 2011 MBoe
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, energy	65,086us-gaap_ProvedDevelopedReservesBOE1	66,140us-gaap_ProvedDevelopedReservesBOE1	69,131us-gaap_ProvedDevelopedReservesBOE1
Proved undeveloped reserves, energy	26,854us-gaap_ProvedUndevelopedReserveBOE1	14,555us-gaap_ProvedUndevelopedReserveBOE1	11,355us-gaap_ProvedUndevelopedReserveBOE1
Balance at end of year, energy	91,940	80,695	80,486	97,651
Changes in proved and undeveloped reserves [Abstract]
Change in PUD reserve	12,300mdu_ChangeInPudReserve
New PUD reserves	21,200mdu_NewPudReserves
Acquired PUD Reserves	3,900mdu_AmountofAcquiredPUDReserves
Converting PUD reserves	3,700mdu_ConvertingPUDreserves
Divested PUD Reserves	6,400mdu_DivestedPUDReserves
Revision to PUD reserves	(2,700)mdu_RevisiontoPUDreserves
Drilling, completion and future development costs [Abstract]
Drilling and completion capital	$ 98.3mdu_DrillingAndCompletionCapital
Future development costs estimated to be spent to develop PUD reserves year one	141.8
Future development costs estimated to be spent to develop PUD reserves year two	157.4
Future development costs estimated to be spent to develop PUD reserves year three	$ 120.9
Oil (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	30,130	31,394	27,412
Proved undeveloped reserves, volume	13,788	9,625	6,041
Balance at end of year, volume	43,918	41,019	33,453	27,005
NGL (MBbls)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	4,217	5,322	5,342
Proved undeveloped reserves, volume	2,970	1,280	1,811
Balance at end of year, volume	7,187	6,602	7,153	7,342
Natural Gas (MMcf)
Proved Developed and Undeveloped Reserves [Abstract]
Proved developed reserves, volume	184,437	176,546	218,259
Proved undeveloped reserves, volume	60,574	21,899	21,019
Balance at end of year, volume	245,011	198,445	239,278	379,827

Amount of acquired PUD reserves.

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- Definition

Change in PUD reserve.

+ References

No definition available.

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- Definition

Converting PUD reserves.

+ References

No definition available.

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- Definition

Amount of divested PUD reserves.

+ References

No definition available.

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- Definition

Drilling and completion capital required to convert PUD reserves into proved developed reserves.

+ References

No definition available.

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- Definition

Future development costs estimated to be spent to develop PUD reserves year one.

+ References

No definition available.

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- Definition

Future development costs estimated to be spent to develop PUD reserves year three.

+ References

No definition available.

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- Definition

Future development costs estimated to be spent to develop PUD reserves year two.

+ References

No definition available.

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- Definition

Amount of new PUD reserves.

+ References

No definition available.

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- Definition

Amount of revision to PUD reserves.

+ References

No definition available.

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- Definition

Energy measure of proved reserve of oil and gas, stated in a unit of energy. For example, but not limited to, barrel of oil equivalents (BOE) or one thousand cubic feet of natural gas equivalent (Mcfe).

+ References

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- Definition

The net quantity of proved reserves as of the balance sheet date. Proved oil reserves are the estimated quantities of crude oil and natural gas liquids which geological and engineering data demonstrate with reasonable certainty to be recoverable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61831-109447

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-10.(a)(2))

-URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e511914-122862

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 10

-Paragraph a

-Subparagraph 2

-Article 4

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Energy measure of interests in proved developed reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

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Net quantities of an enterprise's interests in proved developed reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic 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 4

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61797-109447

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 9

-URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61884-109447

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Energy measure of proved undeveloped reserves of crude oil, including condensate and natural gas liquids, natural gas, synthetic oil and gas, or other nonrenewable natural resource that is intended to be upgraded into synthetic oil and gas.

+ References

No definition available.

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Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves.

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