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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) (Transmission Cost of Service Rates [Member], Texas-New Mexico Power Company [Member], USD $)
In Millions, unless otherwise specified
 6 Months Ended
Mar. 15, 2015
Sep. 07, 2014
Mar. 12, 2014
Sep. 15, 2015
Public Utilities, General Disclosures [Line Items]        
Approved Increase in Rate Base $ 25.2us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount $ 18.2us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount $ 18.1us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount  
Annual Increase in Revenue 4.2pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue 2.9pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue 2.8pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue  
Subsequent Event [Member]        
Public Utilities, General Disclosures [Line Items]        
Approved Increase in Rate Base       27.1us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount
/ dei_LegalEntityAxis
= pnm_TexasNewMexicoPowerCompanyMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_TransmissionCostOfServiceRatesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Annual Increase in Revenue       $ 4.4pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue
/ dei_LegalEntityAxis
= pnm_TexasNewMexicoPowerCompanyMember
/ pnm_ScheduleOfRegulatoryRateMattersAxis
= pnm_TransmissionCostOfServiceRatesMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember