Condensed Statements of Cash Flows (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 615,782	$ 742,193	$ 584,740
Capital expenditures	(972,102)	(909,400)	(872,920)
Net proceeds from sale or disposition of property and other	276,415	124,541	40,110
Investments	709	302	9,725
Net cash used in investing activities	(904,191)	(782,661)	(887,952)
Issuance of long-term debt	606,084	507,924	467,957
Repayment of long-term debt	(368,803)us-gaap_RepaymentsOfLongTermDebt	(423,707)us-gaap_RepaymentsOfLongTermDebt	(138,775)us-gaap_RepaymentsOfLongTermDebt
Proceeds from issuance of common stock	150,060	14,554	88
Dividends paid	(136,712)us-gaap_PaymentsOfDividends	(98,405)us-gaap_PaymentsOfDividends	(159,768)us-gaap_PaymentsOfDividends
Excess tax benefit on stock-based compensation	4,729	0	26
Tax withholding on stock-based compensation	(5,564)	0	0
Net cash provided by financing activities	325,194	36,866	189,628
Increase (decrease) in cash and cash equivalents	36,630	(3,817)	(113,730)
Cash and cash equivalents - beginning of year	45,225	49,042	162,772
Cash and cash equivalents - end of year	81,855	45,225	49,042
MDU Resources Group, Inc. [Member]
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	208,208	188,259	225,968
Capital expenditures	(223,251)	(211,013)	(150,337)
Net proceeds from sale or disposition of property and other	1,552	20,624	1,120
Investments in and advances to subsidiaries	(134,451)	(1,016)	(1,387)
Advances from subsidiaries	64,500	10,000	5,000
Investments	(794)	613	12
Net cash used in investing activities	(292,444)	(180,792)	(145,592)
Issuance of long-term debt	148,959	77,924	76,000
Repayment of long-term debt	(76,432)	(85)	(21)
Proceeds from issuance of common stock	150,060	14,554	88
Dividends paid	(136,712)	(98,405)	(159,768)
Excess tax benefit on stock-based compensation	3,326	0	21
Tax withholding on stock-based compensation	(3,896)	0	0
Net cash provided by financing activities	85,305	(6,012)	(83,680)
Increase (decrease) in cash and cash equivalents	1,069	1,455	(3,304)
Cash and cash equivalents - beginning of year	5,051	3,596	6,900
Cash and cash equivalents - end of year	$ 6,120	$ 5,051	$ 3,596

Investments From and Advances From Subsidiaries.

No definition available.

Name:	mdu_InvestmentsFromAndAdvancesFromSubsidiaries
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Investments in and advances to subsidiaries.

+ References

No definition available.

+ Details

Name:	mdu_InvestmentsInAndAdvancesToSubsidiaries
Namespace Prefix:	mdu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 1

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.1)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3044-108585

+ Details

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

- Definition

Amount of increase (decrease) in cash and cash equivalents. Cash and cash equivalents are the amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Includes effect from exchange rate changes.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3521-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 230

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450594&loc=d3e33268-110906

+ Details

Name:	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Definition

Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 20

-Section 55

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=32706628&loc=d3e11374-113907

+ Details

Name:	us-gaap_ExcessTaxBenefitFromShareBasedCompensationFinancingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow (outflow) from financing activities, including discontinued operations. Financing activity cash flows include obtaining resources from owners and providing them with a return on, and a return of, their investment; borrowing money and repaying amounts borrowed, or settling the obligation; and obtaining and paying for other resources obtained from creditors on long-term credit.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3521-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 26

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3574-108585

+ Details

Name:	us-gaap_NetCashProvidedByUsedInFinancingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow (outflow) from investing activities, including discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3521-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 26

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3574-108585

+ Details

Name:	us-gaap_NetCashProvidedByUsedInInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 24

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3521-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_NetCashProvidedByUsedInOperatingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

The net cash paid (received) associated with the acquisition or disposal of all investments, including securities and other assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3095-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3098-108585

+ Details

Name:	us-gaap_PaymentsForProceedsFromInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cash outflow in the form of capital distributions and dividends to common shareholders, preferred shareholders and noncontrolling interests.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsOfDividends
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

+ References

+ Details

Name:	us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow for purchases of and capital improvements on property, plant and equipment (capital expenditures), software, and other intangible assets.

+ References

+ Details

Name:	us-gaap_PaymentsToAcquireProductiveAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from the additional capital contribution to the entity.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfCommonStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale.

+ References

+ Details

Name:	us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

Name:	us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=us-gaap_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: