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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Mar. 31, 2024
Sep. 30, 2025
Dec. 31, 2024
Jan. 31, 2016
Public Service Company of New Mexico | Four Corners CSA            
Public Utilities, Commitments And Contingencies [Line Items]            
Funds contributed, mine reclamation trust         $ 3.2  
Reclamation trust funding, 2025       $ 0.5    
Reclamation trust funding, 2026       0.5    
Reclamation trust funding, 2027       0.5    
San Juan Generating Station | Coal supply            
Public Utilities, Commitments And Contingencies [Line Items]            
Cash used to support bank letter or credit arrangement           $ 30.3
San Juan Generating Station | Coal supply | NM Capital            
Public Utilities, Commitments And Contingencies [Line Items]            
Requirement to post reclamation bonds $ 118.7     118.7    
Cash used to support bank letter or credit arrangement 30.3     30.3    
Underground | Public Service Company of New Mexico | Loss on long-term purchase commitment            
Public Utilities, Commitments And Contingencies [Line Items]            
Loss contingency accrual 55.0     55.0 52.9  
Underground | San Juan Generating Station | Public Service Company of New Mexico | Loss on long-term purchase commitment            
Public Utilities, Commitments And Contingencies [Line Items]            
Estimate of possible loss 63.7     63.7    
Surface | Public Service Company of New Mexico | Loss on long-term purchase commitment            
Public Utilities, Commitments And Contingencies [Line Items]            
Loss contingency accrual 24.1     24.1 38.3  
Surface | San Juan Generating Station | Public Service Company of New Mexico | Loss on long-term purchase commitment            
Public Utilities, Commitments And Contingencies [Line Items]            
Estimate of possible loss 34.6     34.6    
San Juan Generating Station            
Public Utilities, Commitments And Contingencies [Line Items]            
Reclamation costs   $ 12.1 $ 20.9      
Regulatory disallowance     4.0      
Regulatory disallowance related to other costs   6.1 0.5 $ 0.9    
San Juan Generating Station | Maximum            
Public Utilities, Commitments And Contingencies [Line Items]            
Reclamation costs percentage       9.755%    
San Juan Generating Station | Minimum            
Public Utilities, Commitments And Contingencies [Line Items]            
Reclamation costs percentage       9.096%    
San Juan Generating Station | Public Service Company of New Mexico | Loss on long-term purchase commitment            
Public Utilities, Commitments And Contingencies [Line Items]            
Annual funding post-term reclamation trust         27.3  
Annual funding post-term reclamation trust, remainder of year       $ 6.5    
Annual funding post-term reclamation trust, year one       0.0    
San Juan Generating Station | Underground            
Public Utilities, Commitments And Contingencies [Line Items]            
Increase in regulatory liabilities 0.2 $ 6.0 17.0      
San Juan Generating Station | Surface            
Public Utilities, Commitments And Contingencies [Line Items]            
Reclamation costs         1.6  
Increase in regulatory liabilities $ 0.2     0.9    
Reclamation, maximum collection from customers     $ 100.0   $ 100.0  
San Juan Generating Station and Four Corners | Public Service Company of New Mexico | Mine Reclamation Trust            
Public Utilities, Commitments And Contingencies [Line Items]            
Customer reclamation funding cost       $ 100.0