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Regulatory and Rate Matters - Formula Transmission Rate Case (Details) (Formula Transmission Rate Case [Member], Public Service Company of New Mexico [Member], USD $)
In Millions, unless otherwise specified
 0 Months Ended 3 Months Ended
May 02, 2014
Jun. 03, 2013
Mar. 31, 2015
Mar. 20, 2015
party
Formula Transmission Rate Case [Member] | Public Service Company of New Mexico [Member]
        
Public Utilities, General Disclosures [Line Items]        
Amount of Regulatory Costs Not yet Approved     $ 3.2us-gaap_AmountOfRegulatoryCostsNotYetApproved
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember		  
Public Utilities, Return on Equity     10.81%pnm_PublicUtilitiesReturnonEquity
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember		 10.00%pnm_PublicUtilitiesReturnonEquity
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember
Percentage ownership of EIP transmission line     60.00%pnm_PercentageOwnershipOfEipTransmissionLine
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember		  
Public Utilities, Requested Rate Increase (Decrease), Amount 0.5us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember		 1.3us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember		    
Number of other parties to settlement       5pnm_Numberofotherpartiestosettlement
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember
Public Utilities, Approved Rate Increase (Decrease), Amount     $ 1.3us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_FormulaTransmissionRateCaseMember