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Fair Value of Derivative and Other Financial Instruments - Margin, Notional Amounts and Credit Rating (Details) (PNMR and PNM [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Derivative [Line Items]    
Contractual Liability $ 1,512us-gaap_DerivativeFairValueOfDerivativeLiability $ 1,686us-gaap_DerivativeFairValueOfDerivativeLiability
Existing Cash Collateral 0us-gaap_CollateralAlreadyPostedAggregateFairValue 0us-gaap_CollateralAlreadyPostedAggregateFairValue
Net Exposure $ 117us-gaap_DerivativeNetLiabilityPositionAggregateFairValue $ 167us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Gas related contract [Member] | Commodity Contract [Member] | Fair Value Hedging [Member] | Derivative Long Position [Member]    
Derivative [Line Items]    
Volume positions (Decatherms / MWh) 575,000invest_DerivativeNonmonetaryNotionalAmount
/ pnm_CommoditycontracttypeAxis
= pnm_GasrelatedcontractMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ pnm_DerivativeLongShortPositionAxis
= pnm_DerivativeLongPositionMember
/ dei_LegalEntityAxis
= pnm_PnmrAndPnmMember
650,000invest_DerivativeNonmonetaryNotionalAmount
/ pnm_CommoditycontracttypeAxis
= pnm_GasrelatedcontractMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ pnm_DerivativeLongShortPositionAxis
= pnm_DerivativeLongPositionMember
/ dei_LegalEntityAxis
= pnm_PnmrAndPnmMember
Power related contract [Member] | Commodity Contract [Member] | Fair Value Hedging [Member] | Derivative Short Position [Member]    
Derivative [Line Items]    
Volume positions (Decatherms / MWh) 1,417,913invest_DerivativeNonmonetaryNotionalAmount
/ pnm_CommoditycontracttypeAxis
= pnm_PowerrelatedcontractMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ pnm_DerivativeLongShortPositionAxis
= pnm_DerivativeShortPositionMember
/ dei_LegalEntityAxis
= pnm_PnmrAndPnmMember
1,919,000invest_DerivativeNonmonetaryNotionalAmount
/ pnm_CommoditycontracttypeAxis
= pnm_PowerrelatedcontractMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ pnm_DerivativeLongShortPositionAxis
= pnm_DerivativeShortPositionMember
/ dei_LegalEntityAxis
= pnm_PnmrAndPnmMember