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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
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