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Commitments and Contingencies - Four Corners (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2013 | Mar. 31, 2015 | Aug. 06, 2012 compliance_alternative
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Four Corners [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public Utilities, Jointly Owned Utility Plant, Sale of Ownership Percentage | 48.00%pnm_PublicUtilitiesJointlyOwnedUtilityPlantSaleofOwnershipPercentage / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember |  |
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public Utilities, Number of Compliance alternatives |  |  | 2 pnm_PublicUtilitiesNumberOfComplianceAlternatives / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember
Public Utilities, Plant Requirement to Meet NOx emissions Limit |  | 0.015 pnm_PublicUtilitiesPlantRequirementtoMeetNOxemissionsLimit / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Public Utilities, Plant Requirement to Meet Opacity Limit, Percentage |  | 20.00%pnm_PublicUtilitiesPlantRequirementtoMeetOpacityLimitPercentage / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Public Utilities, Rule Imposes Opacity Limitation on Certain Fugitive Dust Emissions From Coal and Material Handling Operations |  | 20.00%pnm_PublicUtilitiesRuleImposesOpacityLimitationOnCertainFugitiveDustEmissionsFromCoalAndMaterialHandlingOperations / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Loss Contingency, Estimate of Possible Loss |  | $ 83.9 us-gaap_LossContingencyEstimateOfPossibleLoss / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Clean Air Act Related to Post Combustion Controls [Member] | Public Service Company of New Mexico [Member] | Four Corners Units 4 and 5 (Coal) [Member] |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Public Utilities, Ownership Percentage |  | 13.00%pnm_PublicUtilitiesOwnershipPercentage / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_CleanAirActRelatedToPostCombustionControlsMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_FourCornersUnits4And5CoalMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |

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- Definition Reflects the estimated amount of loss from the specified contingency as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349 + Details
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