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Capitalized costs and AD&D related to oil and natural gas producing activities (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Oil and Gas Exploration and Production Industries Disclosures [Abstract] |  |  |
Subject to amortization | $ 3,205,036 mdu_SubjectToAmortization | $ 2,893,010 mdu_SubjectToAmortization | $ 2,531,562 mdu_SubjectToAmortization
Not subject to amortization | 132,141 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative | 124,869 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative | 191,794 us-gaap_CapitalizedCostsOfUnprovedPropertiesExcludedFromAmortizationCumulative
Total capitalized costs | 3,337,177 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesGross | 3,017,879 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesGross | 2,723,356 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesGross
Less accumulated depreciation, depletion and amortization | 1,752,566 us-gaap_CapitalizedCostsAccumulatedDepreciationDepletionAmortizationAndValuationAllowanceForRelatingToOilAndGasProducingActivities | 1,562,116 us-gaap_CapitalizedCostsAccumulatedDepreciationDepletionAmortizationAndValuationAllowanceForRelatingToOilAndGasProducingActivities | 1,383,386 us-gaap_CapitalizedCostsAccumulatedDepreciationDepletionAmortizationAndValuationAllowanceForRelatingToOilAndGasProducingActivities
Net capitalized costs | $ 1,584,611 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesNet | $ 1,455,763 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesNet | $ 1,339,970 us-gaap_CapitalizedCostsOilAndGasProducingActivitiesNet

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- Definition The sum of capitalized costs subject to amortization relating to oil and gas producing activities before accounting for accumulated depreciation. + References No definition available. + Details
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- Definition Amount of accumulated depreciation, depletion, amortization and valuation allowance relating to oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447 + Details
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- Definition The sum of the capitalized costs incurred, as of the balance sheet date, of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs. + References Reference 1: 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.(c)(7)(ii)) -URI http://asc.fasb.org/extlink&oid=27015464&loc=d3e511914-122862 + Details
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- Definition The sum of capitalized costs relating to oil and gas producing activities before accounting for accumulated depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447 + Details
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- Definition Net amount of capitalized costs relating to oil and gas producing activities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e61901-109447 + Details
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