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Summary of the Business and Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 1986 Operating_Leases | Dec. 31, 2004 | Dec. 31, 2003 | Dec. 31, 1985 Operating_Leases
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Debt, Weighted Average Interest Rate | 6.60%us-gaap_DebtWeightedAverageInterestRate | 6.90%us-gaap_DebtWeightedAverageInterestRate | 6.60%us-gaap_DebtWeightedAverageInterestRate |  |  |  |
Interest costs incurred, capitalized | $ 1.6us-gaap_InterestCostsIncurredCapitalized | $ 1.5us-gaap_InterestCostsIncurredCapitalized | $ 1.2us-gaap_InterestCostsIncurredCapitalized |  |  |  |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction, capitalized interest | 4.2us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 3.3us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 3.5us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity | 5.6us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 4.4us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 3.8us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |
Interest costs incurred, capitalized | 1.1us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 1.1us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 0.8us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |
Impairment losses on securities held in the NDT | 4.8us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 3.5us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 4.8us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsAvailableforsaleSecurities / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |
Texas-New Mexico Power Company [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Allowance for funds used during construction, capitalized interest | 0.5us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 0.4us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 0.7us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedInterest / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember |  |  |  |
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity | 0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 0us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 0.6us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionCapitalizedCostOfEquity / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember |  |  |  |
Interest costs incurred, capitalized | $ 0.1us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | $ 0us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | $ 0us-gaap_InterestCostsIncurredCapitalized / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember |  |  |  |
Palo Verde Nuclear Generating Station [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Number of operating leases (in ones) | 8pnm_NumberOfOperatingLeases / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_PaloVerdeNuclearGeneratingStationMember |  |  | 11pnm_NumberOfOperatingLeases / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_PaloVerdeNuclearGeneratingStationMember |  |  |
EIP Transmission Line [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Number of operating leases (in ones) |  |  |  |  |  |  | 2pnm_NumberOfOperatingLeases / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_EipTransmissionLineMember
Ownership percentage acquired (as a percent) |  |  |  |  | 40.00%pnm_BusinessAcquisitionOwnershipPercentageAcquired / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_EipTransmissionLineMember | 60.00%pnm_BusinessAcquisitionOwnershipPercentageAcquired / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_EipTransmissionLineMember |
10.3% Lessor Notes [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Notes receivable, stated percentage rate (as a percent) | 10.30%pnm_NotesReceivableStatedPercentageRate / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = pnm_A103LessorNotesMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |  |  |
10.15% Lessor Notes [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Notes receivable, stated percentage rate (as a percent) | 10.15%pnm_NotesReceivableStatedPercentageRate / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = pnm_A1015LessorNotesMember / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |  |  |  |  |  |
10.25% EIP Lessor Note Maturing in 2012 [Member] | EIP Transmission Line [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |
Notes receivable, stated percentage rate (as a percent) | 10.25%pnm_NotesReceivableStatedPercentageRate / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = pnm_A1025EipLessorNoteMaturingIn2012Member / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_EipTransmissionLineMember |  |  |  |  |  |

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