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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Jan. 31, 2016
PNM | Four Corners CSA        
Public Utilities, Commitments And Contingencies [Line Items]        
Funds contributed, mine reclamation trust   $ 3.2 $ 0.2  
Reclamation trust funding, 2025   1.9    
Reclamation trust funding, year 2026   2.8    
Coal supply | San Juan Generating Station        
Public Utilities, Commitments And Contingencies [Line Items]        
Cash used to support bank letter or credit arrangement       $ 30.3
Coal supply | NM Capital | San Juan Generating Station        
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 | PNM | Loss on long-term purchase commitment        
Public Utilities, Commitments And Contingencies [Line Items]        
Loss contingency accrual 48.6 48.6 26.2  
Underground | PNM | San Juan Generating Station | Loss on long-term purchase commitment        
Public Utilities, Commitments And Contingencies [Line Items]        
Estimate of possible loss 59.7 59.7    
Surface | PNM | Loss on long-term purchase commitment        
Public Utilities, Commitments And Contingencies [Line Items]        
Loss contingency accrual 39.9 39.9 50.0  
Surface | PNM | San Juan Generating Station | Loss on long-term purchase commitment        
Public Utilities, Commitments And Contingencies [Line Items]        
Estimate of possible loss 46.5 46.5    
San Juan Generating Station        
Public Utilities, Commitments And Contingencies [Line Items]        
Reclamation costs   20.9    
Regulatory disallowance 4.5      
San Juan Generating Station | PNM | Loss on long-term purchase commitment        
Public Utilities, Commitments And Contingencies [Line Items]        
Annual funding post-term reclamation trust     $ 2.7  
Annual funding post-term reclamation trust, remainder of year   21.5    
Annual funding post-term reclamation trust, year one   3.6    
Annual funding post-term reclamation trust, year two   3.7    
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]        
Increase in regulatory liabilities 4.0      
Decrease in deferred regulatory assets $ 0.5      
Reclamation, maximum collection from customers   100.0    
San Juan Generating Station and Four Corners | PNM | Mine Reclamation Trust        
Public Utilities, Commitments And Contingencies [Line Items]        
Customer reclamation funding cost   $ 100.0