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Regulatory and Rate Matters - Electric Rate Case (Details) (2014 Electric Rate Case [Member], Public Service Company of New Mexico [Member], USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
2014 Electric Rate Case [Member] | Public Service Company of New Mexico [Member]
  
Public Utilities, General Disclosures [Line Items]  
Public Utilities, Requested Rate Increase (Decrease), Amount $ 107.4us-gaap_PublicUtilitiesRequestedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_A2014ElectricRateCaseMember
Public Utilities, Requested Return on Equity, Percentage 10.50%us-gaap_PublicUtilitiesRequestedReturnOnEquityPercentage
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_A2014ElectricRateCaseMember
Public Utilities, Average customer bill increase percentage 7.69%pnm_PublicUtilitiesAveragecustomerbillincreasepercentage
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_A2014ElectricRateCaseMember
Public Utilities, Percentage of requested rate increase pertaining to infrastructure investments 92.00%pnm_PublicUtilitiesPercentageofrequestedrateincreasepertainingtoinfrastructureinvestments
/ dei_LegalEntityAxis
= pnm_PublicServiceCompanyOfNewMexicoMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_A2014ElectricRateCaseMember
Proposed period of delay 60 days