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Commitments and Contingencies - Litigation and Rulemaking (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Dec. 21, 2011 mw
Public Utilities, Commitments And Contingencies [Line Items] |  |
Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Mercury and Air Toxics Standards |  | 25 pnm_MinimumMegawattCapacityfromCoalandOilFiredElectricGeneratingUnitsunderJurisdictionoftheMercuryandAirToxicsStandards
San Juan Generating Station [Member] |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |
Mercury Removal Rate, Percentage |  | 99.00%pnm_MercuryRemovalRatePercentage / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_SanJuanGeneratingStationMember
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Mercury Control [Member] |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |
Current Annual Mercury Control Costs | $ 0.7 pnm_CurrentAnnualMercuryControlCosts / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_MercuryControlMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_SanJuanGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Public Service Company of New Mexico [Member] | San Juan Generating Station [Member] | Maximum [Member] | Mercury Control [Member] |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |
Contingent Estimated Annual Mercury Control Cost | $ 6.6 pnm_ContingentEstimatedAnnualMercuryControlCost / pnm_CommitmentsAndContingenciesRelatedToEnvironmentAndOtherNatureAxis = pnm_MercuryControlMember / us-gaap_JointlyOwnedUtilityPlantAxis = pnm_SanJuanGeneratingStationMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_RangeAxis = us-gaap_MaximumMember |

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- Definition Contingent Estimated Annual Mercury Control Cost + References No definition available. + Details
Name: | pnm_ContingentEstimatedAnnualMercuryControlCost
Namespace Prefix: | pnm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Current Annual Mercury Control Costs + References No definition available. + Details
Name: | pnm_CurrentAnnualMercuryControlCosts
Namespace Prefix: | pnm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Mercury Removal Rate, Percentage + References No definition available. + Details
Name: | pnm_MercuryRemovalRatePercentage
Namespace Prefix: | pnm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Mercury and Air Toxics Standards + References No definition available. + Details
Name: | pnm_MinimumMegawattCapacityfromCoalandOilFiredElectricGeneratingUnitsunderJurisdictionoftheMercuryandAirToxicsStandards
Namespace Prefix: | pnm_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | pnm_PublicUtilitiesCommitmentsAndContingenciesLineItems
Namespace Prefix: | pnm_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | dei_LegalEntityAxis=pnm_PublicServiceCompanyOfNewMexicoMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_JointlyOwnedUtilityPlantAxis=pnm_SanJuanGeneratingStationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |