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Commitments and Contingencies (Commitments and Contingencies Related to the Environment) (Details) (USD $)
In Millions, unless otherwise specified
 0 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended 3 Months Ended
Dec. 31, 2011
mw
Sep. 21, 2011
EPA [Member]
Mar. 31, 2012
EPA [Member]
Mar. 31, 2012
WildEarth Guardians filed an action to challenge EPA's rule in the Tenth Circuit [Member]
San Juan Generating Station [Member]
EPA [Member]
Mar. 31, 2012
Clean Air Act related to Regional Haze [Member]
Mar. 31, 2012
Sierra Club Allegations [Member]
San Juan Generating Station [Member]
Jan. 31, 2010
Public Service Company of New Mexico [Member]
San Juan Generating Station And Four Corners [Member]
Minimum [Member]
opp
Jan. 31, 2010
Public Service Company of New Mexico [Member]
San Juan Generating Station And Four Corners [Member]
Maximum [Member]
opp
Mar. 31, 2012
Public Service Company of New Mexico [Member]
San Juan Generating Station [Member]
Dec. 31, 2011
Public Service Company of New Mexico [Member]
San Juan Generating Station [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Nuclear Spent Fuel And Waste Disposal [Member]
Palo Verde Nuclear Generating Station [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Nuclear Spent Fuel And Waste Disposal [Member]
Palo Verde Nuclear Generating Station [Member]
Other Deferred Credits [Member]
Dec. 31, 2011
Public Service Company of New Mexico [Member]
Nuclear Spent Fuel And Waste Disposal [Member]
Palo Verde Nuclear Generating Station [Member]
Other Deferred Credits [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act related to SCR [Member]
San Juan Generating Station [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act, SNCR [Member]
San Juan Generating Station [Member]
Installation Costs [Member]
Sep. 21, 2011
Public Service Company of New Mexico [Member]
Clean Air Act, SCR [Member]
San Juan Generating Station [Member]
Selective_Catalytic_Reduction
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act, SCR [Member]
San Juan Generating Station [Member]
Operating Costs [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act, SCR [Member]
San Juan Generating Station [Member]
Operating Costs [Member]
Share of project costs to be incurred in 2013 [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act related to Post-Combustion Controls [Member]
Four Corners [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Clean Air Act related to Post-Combustion Controls [Member]
Four Corners [Member]
EPA [Member]
Apr. 06, 2012
Public Service Company of New Mexico [Member]
National Ambient Air Quality Standards [Member]
San Juan Generating Station And Four Corners [Member]
Application for an amendment to its air permit [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Mercury Control [Member]
San Juan Generating Station [Member]
May 17, 2010
Public Service Company of New Mexico [Member]
Mercury Control [Member]
San Juan Generating Station [Member]
Maximum [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Sierra Club Allegations [Member]
San Juan Generating Station [Member]
Mar. 31, 2012
Public Service Company of New Mexico [Member]
Cooling Water Intake Structures [Member]
EPA [Member]
Loss Contingencies [Line Items]                                                  
Public Utilities, Period of required selective catalytic reduction to be installed   5 years                                              
Loss Contingency, Estimate of Possible Loss                     $ 42.8       $ 77.0       $ 69.0       $ 6.0    
Public Utilities, Estimate cost of equipment installation modifications to reduce fugitive emissions                                         140        
Public Utilities, Estimate cost of equipment installation modifications to reduce fugitive emissions, percentage share of cost to purchase equipment                                         46.30%        
Jointly Owned Utility Plant, Proportionate Ownership Share                 46.30%                                
Loss Contingency Accrual, at Carrying Value                       13.9 14.5                        
Public Utilities, Potential to emit tons per year of visibility impairing pollution, maximum         250                                        
Public Utilities, Number of selective catalytic reduction installations that might need to be installed                               4                  
Loss Contingency, Range of Possible Loss, Minimum                           750.0     18.0 85.0              
Loss Contingency, Range of Possible Loss, Maximum                           1,000.0     27.0 115.0              
Public Utilities, Government Standard Emission Limit (pounds per MMBTU, parts per million)             0.060 0.070                                  
Loss Contingency, Loss in Period           $ 10.2                               $ 0.6   $ 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, 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