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Commitments and Contingencies (Commitments and Contingencies Related to the Environment) (Details) (USD $)
In Millions, unless otherwise specified
 0 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 3 Months Ended 6 Months Ended
Dec. 31, 2011
mw
Sep. 21, 2011
EPA [Member]
Jun. 30, 2012
EPA [Member]
Jun. 30, 2012
WildEarth Guardians filed an action to challenge EPA's rule in the Tenth Circuit [Member]
San Juan Generating Station [Member]
EPA [Member]
Jun. 30, 2012
Clean Air Act related to Regional Haze [Member]
Jun. 30, 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
Jun. 30, 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]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Nuclear Spent Fuel And Waste Disposal [Member]
Palo Verde Nuclear Generating Station [Member]
Jun. 30, 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]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Clean Air Act Related to Scr [Member]
San Juan Generating Station [Member]
Apr. 30, 2012
Public Service Company of New Mexico [Member]
Clean Air Act Related to Scr [Member]
San Juan Generating Station [Member]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Clean Air Act, SNCR [Member]
San Juan Generating Station [Member]
Installation Costs [Member]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Clean Air Act Related to Post Combustion Controls [Member]
Four Corners [Member]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Clean Air Act Related to Post Combustion Controls [Member]
Four Corners [Member]
EPA [Member]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
National Ambient Air Quality Standards [Member]
San Juan Generating Station [Member]
Jun. 30, 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]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Sierra Club Allegations [Member]
San Juan Generating Station [Member]
Jun. 30, 2012
Public Service Company of New Mexico [Member]
Cooling Water Intake Structures [Member]
EPA [Member]
Jun. 30, 2012
Previously Estimated [Member]
Public Service Company of New Mexico [Member]
Clean Air Act, SCR [Member]
San Juan Generating Station [Member]
Operating Costs [Member]
Jun. 30, 2012
Previously Estimated [Member]
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]
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        
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                                        
Loss Contingency, Range of Possible Loss, Minimum                           750 750                 18 85
Loss Contingency, Range of Possible Loss, Maximum                           805 1,000       135         27 115
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