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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2025
Sep. 30, 2024
Jun. 30, 2024
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        
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        
Cash used to support bank letter or credit arrangement 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 53.5     $ 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 62.7        
Surface | Public Service Company of New Mexico | Loss on long-term purchase commitment          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency accrual 34.4     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 46.5        
San Juan Generating Station          
Public Utilities, Commitments And Contingencies [Line Items]          
Reclamation costs   $ 20.9      
Regulatory disallowance   4.0      
Regulatory disallowance related to other costs   0.5      
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 1.6        
Annual funding post-term reclamation trust, year one 1.0        
Annual funding post-term reclamation trust, year two 0.4        
San Juan Generating Station | Underground          
Public Utilities, Commitments And Contingencies [Line Items]          
Increase in regulatory liabilities   17.0      
San Juan Generating Station | Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Reclamation costs       1.6  
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