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Preferred Stocks (Details) (USD $) In Thousands, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Class of Stock [Line Items] |  |  |
Preferred stock redemption price upon liquidation (in dollars per share) | $ 100 us-gaap_PreferredStockRedemptionPricePerShare |  |
Preferred stocks | $ 15,000 us-gaap_PreferredStockValue | $ 15,000 us-gaap_PreferredStockValue |
Cumulative Preferred Stock, Par Value $100 Issuable in Series [Member] |  |  |
Class of Stock [Line Items] |  |  |
Authorized preferred cumulative shares (in shares) | 500,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockWithParValueMember | 500,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockWithParValueMember |
Cumulative preferred shares, par value (in dollars per share) | $ 100.00 us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockWithParValueMember | $ 100.00 us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockWithParValueMember |
Preferred stock, series 4.50% [Member] |  |  |
Class of Stock [Line Items] |  |  |
Preferred stock redemption price at the option of the company (in dollars per share) | $ 105 mdu_PreferredStockRedemptionPriceOptionOfCompany / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member |  |
Preferred stocks | 10,000 us-gaap_PreferredStockValue / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | 10,000 us-gaap_PreferredStockValue / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member |
Preferred shares outstanding (in shares) | 100,000 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | 100,000 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member |
Preferred stock, $100 par value | 4.50%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | 4.50%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | 4.50%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member
Preferred stock, dividends per share, declared | $ 4.50 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | $ 4.50 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member | $ 4.50 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries45Member
Preferred stock, series 4.70% [Member] |  |  |
Class of Stock [Line Items] |  |  |
Preferred stock redemption price at the option of the company (in dollars per share) | $ 102 mdu_PreferredStockRedemptionPriceOptionOfCompany / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member |  |
Preferred stocks | $ 5,000 us-gaap_PreferredStockValue / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | $ 5,000 us-gaap_PreferredStockValue / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member |
Preferred shares outstanding (in shares) | 50,000 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | 50,000 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member |
Preferred stock, $100 par value | 4.70%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | 4.70%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | 4.70%us-gaap_PreferredStockDividendRatePercentage / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member
Preferred stock, dividends per share, declared | $ 4.70 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | $ 4.70 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member | $ 4.70 us-gaap_PreferredStockDividendsPerShareDeclared / us-gaap_StatementClassOfStockAxis = mdu_PreferredStockSeries470Member
Cumulative preferred stock A, without par value, issuable in series [Member] |  |  |
Class of Stock [Line Items] |  |  |
Authorized preferred cumulative shares (in shares) | 1,000,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember | 1,000,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = us-gaap_PreferredClassAMember |
Cumulative preference stock , without par value, issuable in series [Member] |  |  |
Class of Stock [Line Items] |  |  |
Authorized preferred cumulative shares (in shares) | 500,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = mdu_PreferenceStockMember | 500,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = mdu_PreferenceStockMember |

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- Definition The redemption price per share at which the preferred stock of an entity that has priority over common stock in the distribution of dividends may be redeemed at the option of the company with certain limitation + References No definition available. + Details
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- Definition The percentage rate used to calculate dividend payments on preferred stock. + References No definition available. + Details
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- Definition Aggregate dividends declared during the period for each share of preferred stock outstanding. + References No definition available. + Details
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- Definition Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition The price per share at which the preferred stock of an entity that has priority over common stock in the distribution of dividends and in the event of liquidation of the entity is redeemed or may be called at. The redemption features of this preferred stock are solely within the control of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644 + Details
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- Definition The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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