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Summary of the Business and Significant Accounting Policies (Inventories/Depreciation and Amortization) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 63,628us-gaap_InventoryNet | 67,223us-gaap_InventoryNet |
Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 17,525us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember | 24,872us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember |
Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 46,103us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember | 42,351us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember |
Public Service Company of New Mexico [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 60,859us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember | 64,520us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember |
Public Service Company of New Mexico [Member] | Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 17,525us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember | 24,872us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember |
Public Service Company of New Mexico [Member] | Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 43,334us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember | 39,648us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember |
Public Service Company of New Mexico [Member] | Electric plant [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used (as a percent) | 2.26%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricityGenerationPlantNonNuclearMember | 2.27%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricityGenerationPlantNonNuclearMember | 2.25%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = us-gaap_ElectricityGenerationPlantNonNuclearMember
Public Service Company of New Mexico [Member] | Common, intangible, and general plant [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used (as a percent) | 4.64%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_CommonIntangibleAndGeneralPlantMember | 4.87%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_CommonIntangibleAndGeneralPlantMember | 5.35%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_PublicServiceCompanyOfNewMexicoMember / us-gaap_PublicUtilityAxis = pnm_CommonIntangibleAndGeneralPlantMember
Texas-New Mexico Power Company [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 2,769us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 2,703us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember |
Depreciation average rates used (as a percent) | 3.59%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 3.66%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember | 3.56%us-gaap_PublicUtilitiesPropertyPlantAndEquipmentDisclosureOfCompositeDepreciationRateForPlantsInService / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember
Texas-New Mexico Power Company [Member] | Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 0us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember | 0us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / us-gaap_PublicUtilitiesInventoryAxis = us-gaap_PublicUtilitiesInventoryCoalMember |
Texas-New Mexico Power Company [Member] | Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 2,769us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember | 2,703us-gaap_InventoryNet / dei_LegalEntityAxis = pnm_TexasNewMexicoPowerCompanyMember / us-gaap_PublicUtilitiesInventoryAxis = pnm_PublicUtilitiesInventoryMaterialsAndSuppliesMember |

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- Definition Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6386567&loc=d3e3927-108312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 + Details
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