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Quarterly Financial Information (Unaudited) (Details) (USD $)
Share data in Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]                      
Operating revenues $ 1,163,204,000us-gaap_Revenues $ 1,370,455,000us-gaap_Revenues $ 1,094,046,000us-gaap_Revenues $ 1,042,853,000us-gaap_Revenues $ 1,184,423,000us-gaap_Revenues $ 1,285,782,000us-gaap_Revenues $ 1,060,595,000us-gaap_Revenues $ 931,604,000us-gaap_Revenues $ 4,670,558,000us-gaap_Revenues $ 4,462,404,000us-gaap_Revenues $ 4,075,431,000us-gaap_Revenues
Total operating expenses 1,053,582,000us-gaap_OperatingExpenses 1,193,690,000us-gaap_OperatingExpenses 995,112,000us-gaap_OperatingExpenses 939,949,000us-gaap_OperatingExpenses 1,037,306,000us-gaap_OperatingExpenses 1,135,909,000us-gaap_OperatingExpenses 969,217,000us-gaap_OperatingExpenses 827,073,000us-gaap_OperatingExpenses 4,182,333,000us-gaap_OperatingExpenses 3,969,505,000us-gaap_OperatingExpenses 4,056,224,000us-gaap_OperatingExpenses
Operating income 109,622,000us-gaap_OperatingIncomeLoss 176,765,000us-gaap_OperatingIncomeLoss 98,934,000us-gaap_OperatingIncomeLoss 102,904,000us-gaap_OperatingIncomeLoss 147,117,000us-gaap_OperatingIncomeLoss 149,873,000us-gaap_OperatingIncomeLoss 91,378,000us-gaap_OperatingIncomeLoss 104,531,000us-gaap_OperatingIncomeLoss 488,225,000us-gaap_OperatingIncomeLoss 492,899,000us-gaap_OperatingIncomeLoss 19,207,000us-gaap_OperatingIncomeLoss
Income from continuing operations 80,079,000us-gaap_IncomeLossFromContinuingOperations 102,118,000us-gaap_IncomeLossFromContinuingOperations 52,780,000us-gaap_IncomeLossFromContinuingOperations 56,184,000us-gaap_IncomeLossFromContinuingOperations 91,348,000us-gaap_IncomeLossFromContinuingOperations 84,550,000us-gaap_IncomeLossFromContinuingOperations 46,392,000us-gaap_IncomeLossFromContinuingOperations 56,592,000us-gaap_IncomeLossFromContinuingOperations 291,161,000us-gaap_IncomeLossFromContinuingOperations 278,882,000us-gaap_IncomeLossFromContinuingOperations (14,321,000)us-gaap_IncomeLossFromContinuingOperations
Income (loss) from discontinued operations, net of tax 2,672,000us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax 3,000us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax 547,000us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (45,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (58,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (118,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (59,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax (77,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax 3,177,000us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax [1] (312,000)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax [1] 13,567,000us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax [1]
Net income (loss) attributable to parent 84,256,000us-gaap_NetIncomeLoss 103,209,000us-gaap_NetIncomeLoss 54,106,000us-gaap_NetIncomeLoss 56,662,000us-gaap_NetIncomeLoss 91,450,000us-gaap_NetIncomeLoss 84,456,000us-gaap_NetIncomeLoss 46,512,000us-gaap_NetIncomeLoss 56,515,000us-gaap_NetIncomeLoss      
Earnings (loss) per common share - basic [Abstract]                      
Earnings before discontinued operations $ 0.42us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.53us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.28us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.30us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.48us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.45us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.25us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 0.30us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 1.53us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ 1.47us-gaap_IncomeLossFromContinuingOperationsPerBasicShare $ (0.08)us-gaap_IncomeLossFromContinuingOperationsPerBasicShare
Discontinued operations, net of tax $ 0.01us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.02us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare $ 0.07us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerBasicShare
Earnings (loss) per common share - basic $ 0.43us-gaap_EarningsPerShareBasic $ 0.53us-gaap_EarningsPerShareBasic $ 0.28us-gaap_EarningsPerShareBasic $ 0.30us-gaap_EarningsPerShareBasic $ 0.48us-gaap_EarningsPerShareBasic $ 0.45us-gaap_EarningsPerShareBasic $ 0.25us-gaap_EarningsPerShareBasic $ 0.30us-gaap_EarningsPerShareBasic $ 1.55us-gaap_EarningsPerShareBasic $ 1.47us-gaap_EarningsPerShareBasic $ (0.01)us-gaap_EarningsPerShareBasic
Earnings per common share - diluted [Abstract]                      
Earnings before discontinued operations $ 0.42us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.53us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.28us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.30us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.48us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.44us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.24us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 0.30us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 1.53us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ 1.47us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare $ (0.08)us-gaap_IncomeLossFromContinuingOperationsPerDilutedShare
Discontinued operations, net of tax $ 0.01us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.02us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.00us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare $ 0.07us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxPerDilutedShare
Earnings (loss) per common share - diluted $ 0.43us-gaap_EarningsPerShareDiluted $ 0.53us-gaap_EarningsPerShareDiluted $ 0.28us-gaap_EarningsPerShareDiluted $ 0.30us-gaap_EarningsPerShareDiluted $ 0.48us-gaap_EarningsPerShareDiluted $ 0.44us-gaap_EarningsPerShareDiluted $ 0.24us-gaap_EarningsPerShareDiluted $ 0.30us-gaap_EarningsPerShareDiluted $ 1.55us-gaap_EarningsPerShareDiluted $ 1.47us-gaap_EarningsPerShareDiluted $ (0.01)us-gaap_EarningsPerShareDiluted
Weighted average common shares outstanding - basic 194,136us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 193,949us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 192,060us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 189,820us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,929us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,831us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,831us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,831us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 192,507us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,855us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 188,826us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Weighted average common shares outstanding - diluted 194,219us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 194,300us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 192,659us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 190,432us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 189,766us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 189,638us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 189,463us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 189,222us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 192,587us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 189,693us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 188,826us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Unrealized gain (loss) on commodity derivatives 4,100,000mdu_UnrealizedGainLossonCommodityContractsNetofTax 18,100,000mdu_UnrealizedGainLossonCommodityContractsNetofTax (3,300,000)mdu_UnrealizedGainLossonCommodityContractsNetofTax (4,300,000)mdu_UnrealizedGainLossonCommodityContractsNetofTax (500,000)mdu_UnrealizedGainLossonCommodityContractsNetofTax (7,900,000)mdu_UnrealizedGainLossonCommodityContractsNetofTax 8,200,000mdu_UnrealizedGainLossonCommodityContractsNetofTax (3,700,000)mdu_UnrealizedGainLossonCommodityContractsNetofTax      
MEPP withdrawal liability, after tax 8,400,000mdu_EstimatedMultiemployerPlanWithdrawalObligationAfterTax               8,400,000mdu_EstimatedMultiemployerPlanWithdrawalObligationAfterTax    
Asset impairment charges             9,000,000us-gaap_AssetImpairmentCharges     9,000,000us-gaap_AssetImpairmentCharges 1,700,000us-gaap_AssetImpairmentCharges
Reversal of an arbitration charge after tax         $ 1,500,000mdu_ReversalOfArbitrationChargeAfterTax         $ 1,500,000mdu_ReversalOfArbitrationChargeAfterTax $ 15,000,000mdu_ReversalOfArbitrationChargeAfterTax
[1] Reflected in the Other category.