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Estimated discounted future cash flows of total proved reserves (Details 6) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
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Oil and Gas Exploration and Production Industries Disclosures [Abstract] |  |  |  |
Future cash inflows | $ 5,185,500 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesCashInflows | $ 4,507,000 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesCashInflows | $ 3,696,200 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesCashInflows |
Future production costs | 1,856,900 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts | 1,734,800 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts | 1,536,500 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesProductionCosts |
Future development costs | 570,200 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesDevelopmentCosts | 403,000 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesDevelopmentCosts | 301,600 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesDevelopmentCosts |
Future net cash flows before income taxes | 2,758,400 mdu_DiscountedFutureNetCashFlowsBeforeIncomeTaxes | 2,369,200 mdu_DiscountedFutureNetCashFlowsBeforeIncomeTaxes | 1,858,100 mdu_DiscountedFutureNetCashFlowsBeforeIncomeTaxes |
Future income tax expense | 686,100 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense | 545,200 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense | 304,900 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesIncomeTaxExpense |
Future net cash flows | 2,072,300 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows | 1,824,000 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows | 1,553,200 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesNetCashFlows |
10% annual discount for estimated timing of cash flows | 997,400 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows | 810,000 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows | 669,800 us-gaap_FutureNetCashFlowsRelatingToProvedOilAndGasReservesTenPercentAnnualDiscountForEstimatedTimingOfCashFlows |
Discounted future net cash flows relating to proved oil, NGL and natural gas reserves | $ 1,074,900 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 1,014,000 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 883,400 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves | $ 978,800 us-gaap_StandardizedMeasureOfDiscountedFutureNetCashFlowsRelatingToProvedOilAndGasReserves

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- Definition Amount of estimated future accumulated cash inflow from the sale of proved oil and 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 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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- Definition Amount of estimated future accumulated cash outflow for costs to be incurred in developing proved oil and 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 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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- Definition Amount of estimated future accumulated cash outflow for income tax, based on the future income less future expense derived from production and sale of proved oil and 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 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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- Definition Amount, after future accumulated cash outflow for development and production costs and future income tax, of estimated future accumulated cash inflow from the sale of proved oil and 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 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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- Definition Amount of estimated future accumulated cash outflow for costs to be incurred in producing proved oil and 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 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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- Definition Amount of discount on future accumulated cash inflow, after deducting future accumulated cash outflow for development and production costs and future income tax, from sale of proved oil and gas reserves, using a discount rate of ten percent a year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 30 -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62424-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 31 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=8451039&loc=d3e62455-109447 + Details
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