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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) (USD $) | Mar. 31, 2015 | Apr. 01, 2014 | Jan. 22, 2015 Allotment_Parcel | Apr. 02, 2014 Allotment_Parcel | Jan. 06, 2014 Allotment_Parcel | Sep. 30, 2012 Allotment_Parcel
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Continuous Highwall Mining [Member] | San Juan Generating Station [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Proposed Retroactive Surface Mining Royalty Rate | 12.50%pnm_PublicUtilitiesProposedRetroactiveSurfaceMiningRoyaltyRate / pnm_JointlyOwnedGeneratingFacilitiesAxis = pnm_SanJuanGeneratingStationMember / us-gaap_OtherCommitmentsAxis = pnm_ContinuousHighwallMiningMember |  |  |  |  |
Public Utilities, Current Surface Mining Royalty Rate applied between 2000 and 2003 | 8.00%pnm_PublicUtilitiesCurrentSurfaceMiningRoyaltyRateappliedbetween2000and2003 / pnm_JointlyOwnedGeneratingFacilitiesAxis = pnm_SanJuanGeneratingStationMember / us-gaap_OtherCommitmentsAxis = pnm_ContinuousHighwallMiningMember |  |  |  |  |
Public Utilities, Estimated Underpaid Surface Mining Royalties under proposed rate change | $ 5,000,000 pnm_PublicUtilitiesEstimatedUnderpaidSurfaceMiningRoyaltiesunderproposedratechange / pnm_JointlyOwnedGeneratingFacilitiesAxis = pnm_SanJuanGeneratingStationMember / us-gaap_OtherCommitmentsAxis = pnm_ContinuousHighwallMiningMember |  |  |  |  |
Public Utilities, PNM Share Estimated Underpaid Surface Mining Royalties under proposed rate change | 46.30%pnm_PublicUtilitiesPNMShareEstimatedUnderpaidSurfaceMiningRoyaltiesunderproposedratechange / pnm_JointlyOwnedGeneratingFacilitiesAxis = pnm_SanJuanGeneratingStationMember / us-gaap_OtherCommitmentsAxis = pnm_ContinuousHighwallMiningMember |  |  |  |  |
NMTRD Coal Severance Tax [Member] | Four Corners [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Assessed Coal Severance Surtax Penalty and Interest | 30,000,000 pnm_PublicUtilitiesAssessedCoalSeveranceSurtaxPenaltyandInterest / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / us-gaap_OtherCommitmentsAxis = pnm_NMTRDCoalSeveranceTaxMember |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Ownership Percentage in Nuclear Reactor | 10.20%pnm_PublicUtilitiesOwnershipPercentageInNuclearReactor / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Utilities, Maximum Potential Assessment Per Incident | 38,900,000 pnm_PublicUtilitiesMaximumPotentialAssessmentPerIncident / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Utilities, Annual Payment Limitation Related to Incident | 5,700,000 pnm_PublicUtilitiesAnnualPaymentLimitationRelatedToIncident / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Utilities, Aggregate Amount of All Risk Insurance | 2,750,000,000 pnm_PublicUtilitiesAggregateAmountOfAllRiskInsurance / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Utilities, Maximum Amount under Nuclear Electric Insurance Limited | 5,400,000 pnm_PublicUtilitiesMaximumAmountunderNuclearElectricInsuranceLimited / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Maximum [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage | 13,600,000,000 pnm_PublicUtilitiesLiabilityInsuranceCoverage / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Public Utilities, Liability Insurance Coverage Sublimit |  | 2,250,000,000 pnm_PublicUtilitiesLiabilityInsuranceCoverageSublimit / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |
Public Service Company of New Mexico [Member] | NMTRD Coal Severance Tax [Member] | Four Corners [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, PNM Share Assessed Coal Severance Surtax Penalty and Interest to pass through FFPAC | 9.40%pnm_PublicUtilitiesPNMShareAssessedCoalSeveranceSurtaxPenaltyandInteresttopassthroughFFPAC / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_OtherCommitmentsAxis = pnm_NMTRDCoalSeveranceTaxMember |  |  |  |  |
Public Service Company of New Mexico [Member] | Navajo Nation Allottee Matters [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Number of landowners claiming to be Navajo allottees |  |  |  |  |  | 43 pnm_NumberoflandownersclaimingtobeNavajoallottees / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_OtherCommitmentsAxis = pnm_NavajoNationAllotteeMattersMember
Number of allotment parcels' appraisal requested for review |  |  |  |  | 58 pnm_Numberofallotmentparcelsappraisalrequestedforreview / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_OtherCommitmentsAxis = pnm_NavajoNationAllotteeMattersMember |
Number of allotments where landowners are revoking rights of way renewal consents |  |  | 10 pnm_Numberofallotmentswherelandownersarerevokingrightsofwayrenewalconsents / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_OtherCommitmentsAxis = pnm_NavajoNationAllotteeMattersMember | 6 pnm_Numberofallotmentswherelandownersarerevokingrightsofwayrenewalconsents / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_OtherCommitmentsAxis = pnm_NavajoNationAllotteeMattersMember |  |
Public Service Company of New Mexico [Member] | Commercial Providers [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage | 375,000,000 pnm_PublicUtilitiesLiabilityInsuranceCoverage / pnm_InsuranceCarrierAxis = pnm_CommercialProvidersMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |
Public Service Company of New Mexico [Member] | Industry Wide Retrospective Assessment Program [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage | $ 13,200,000,000 pnm_PublicUtilitiesLiabilityInsuranceCoverage / pnm_InsuranceCarrierAxis = pnm_IndustryWideRetrospectiveAssessmentProgramMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_PaloVerdeNuclearGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_NuclearPlantMember |  |  |  |  |

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