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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
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Mar. 15, 2015 | Sep. 07, 2014 | Mar. 12, 2014 | Sep. 15, 2015
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base | $ 25.2 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount | $ 18.2 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount | $ 18.1 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount |
Annual Increase in Revenue | 4.2 pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue | 2.9 pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue | 2.8 pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue |
Subsequent Event [Member] |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base |  |  |  | 27.1 us-gaap_PublicUtilitiesApprovedRateIncreaseDecreaseAmount / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / pnm_ScheduleOfRegulatoryRateMattersAxis = pnm_TransmissionCostOfServiceRatesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Annual Increase in Revenue |  |  |  | $ 4.4 pnm_PublicUtilitiesIncreaseAnnualTransmissionServiceRevenue / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / pnm_ScheduleOfRegulatoryRateMattersAxis = pnm_TransmissionCostOfServiceRatesMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Public Utilities, Increase Annual Transmission Service Revenue + References No definition available. + Details
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- Definition Amount of public utility's approved rate increase (decrease) by regulatory agency. + References No definition available. + Details
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