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Commitments and Contingencies (Commitments and Contingencies Related to the Environment) (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  |  |  |  | 12 Months Ended |  |  | 12 Months Ended |  |  |  | 0 Months Ended | 9 Months Ended
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Dec. 31, 2011 mw | Sep. 30, 2012 San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2011 San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Nuclear Spent Fuel And Waste Disposal [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Nuclear Spent Fuel And Waste Disposal [Member] Other Deferred Credits [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2011 Nuclear Spent Fuel And Waste Disposal [Member] Other Deferred Credits [Member] Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Clean Air Act related to Regional Haze [Member] state | Sep. 30, 2012 Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member] | Aug. 06, 2012 Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member] compliance_alternative | Sep. 30, 2012 Mercury Control [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Apr. 12, 2012 Sierra Club Allegations [Member] San Juan Generating Station [Member] | Apr. 12, 2012 Sierra Club Allegations [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Jan. 31, 2010 Minimum [Member] San Juan Generating Station And Four Corners [Member] Public Service Company of New Mexico [Member] opp | Jan. 31, 2010 Maximum [Member] San Juan Generating Station And Four Corners [Member] Public Service Company of New Mexico [Member] opp | Sep. 30, 2012 Maximum [Member] Mercury Control [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Previously Estimated [Member] Operating Costs [Member] Clean Air Act, SCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Oct. 31, 2012 Subsequent Event [Member] Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] Minimum [Member] Clean Air Act, SNCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Oct. 31, 2012 Subsequent Event [Member] Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] Maximum [Member] Clean Air Act, SNCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Share of project costs to be incurred in 2013 [Member] Previously Estimated [Member] Operating Costs [Member] Clean Air Act, SCR [Member] San Juan Generating Station [Member] Public Service Company of New Mexico [Member] | Sep. 21, 2011 Environmental Protection Agency [Member] | Sep. 30, 2012 Environmental Protection Agency [Member] | Sep. 30, 2012 Environmental Protection Agency [Member] WildEarth Guardians filed an action to challenge EPA's rule in the Tenth Circuit [Member] San Juan Generating Station [Member] | Sep. 30, 2012 Environmental Protection Agency [Member] Clean Air Act Related to Post Combustion Controls [Member] Four Corners [Member] Public Service Company of New Mexico [Member] | Sep. 30, 2012 Environmental Protection Agency [Member] Cooling Water Intake Structures [Member] Public Service Company of New Mexico [Member]
Public Utilities, Commitments and Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Number of Compliance alternatives |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Company Share Estimated On Site Interim Storage Costs |  |  |  | $ 42.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued Interim Spent Nuclear Fuel Storage Costs |  |  |  |  | 13.8 | 14.5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Number of States To Address Regional Haze |  |  |  |  |  |  | 50 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Potential to emit tons per year of visibility impairing pollution, maximum |  |  |  |  |  |  | 250 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated Total Capital Cost If Requirement Occured |  |  |  |  |  |  |  | 69.0 |  |  |  |  |  |  |  |  | 824.0 | 910.0 |  |  |  |  |  |
Estimated Installation Capital Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 85 | 90 |  |  |  |  |  |
Estimated Portion of Total Capital Costs if Requirement Occured |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 105 | 110 |  |  |  |  |  |
Jointly Owned Utility Plant, Proportionate Ownership Share |  | 46.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated 2012 Company Share of Capital Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  |  |
Estimated 2013 Company Share of Capital Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 102 |  |  |  |  |
Public Utilities, Government Standard Emission Limit |  |  |  |  |  |  |  | 0.015 |  |  |  |  | 0.060 | 0.070 |  |  |  |  |  |  |  |  |  |
Public Utilitites, Plant Requirment to Meet Opacity Limit, Percentage |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Rule Imposes Opacity Limitation on Certain Fugitive Dust Emissions From Coal and Material Handling Operations |  |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current Annual Mercury Control Costs |  |  |  |  |  |  |  |  |  | 0.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contingent Estimated Annual Mercury Control Cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.0 |  |  |  |  |  |  |  |  |
Estimated Total Litigation Settlement Cost |  |  |  |  |  |  |  |  |  |  | 10.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated Company Share Litigation Settlement Cost |  |  |  |  |  |  |  |  |  |  |  | $ 4.5 |  |  |  |  |  |  |  |  |  |  |  |
Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Merucy and Air Toxics Standards | 25 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mercury Removal Rate, Percentage |  |  | 0.99 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Original Compliance Period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |
Public Utilities, Period of required selective catalytic reduction to be installed |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |  |  |
Public Utilities, Proposed Seeking Shorter Compliance Period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |
Public Utilities, Number of years after issuance of final determination to achieve compliance with requirements |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |
Public Utilites, Number of days to repond to request |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90 days |  |  |
Public Utilities, Proposed rule requires existing facilities to comply with the impingement mortality requirements, maximum time to comply |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 years

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- Definition Mercury Removal Rate, Percentage + References No definition available. + Details
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- Definition Minimum Megawatt Capacity from Coal and Oil-Fired Electric Generating Units under Jurisdiction of the Merucy and Air Toxics Standards + References No definition available. + Details
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- Definition Public Utilities, Period of required selective catalytic reduction to be installed + References No definition available. + Details
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