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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Jan. 31, 2016
PNM | Loss on long-term purchase commitment | Surface        
Public Utilities, Commitments And Contingencies [Line Items]        
Loss contingency accrual $ 44.5 $ 50.0    
PNM | Loss on long-term purchase commitment | Underground        
Public Utilities, Commitments And Contingencies [Line Items]        
Loss contingency accrual 47.9 26.2    
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface        
Public Utilities, Commitments And Contingencies [Line Items]        
Estimate of possible loss 51.4      
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground        
Public Utilities, Commitments And Contingencies [Line Items]        
Estimate of possible loss 59.7      
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      
Cash used to support bank letter or credit arrangement 30.3      
Four Corners CSA | PNM        
Public Utilities, Commitments And Contingencies [Line Items]        
Funds contributed, mine reclamation trust   0.2 $ 2.4  
Reclamation trust funding, remainder of year 3.1      
Reclamation trust funding, 2025 1.3      
Reclamation trust funding, year 2026 1.5      
San Juan Generating Station        
Public Utilities, Commitments And Contingencies [Line Items]        
Reclamation costs 20.9      
Regulatory disallowance 4.5      
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 | Underground        
Public Utilities, Commitments And Contingencies [Line Items]        
Increase in regulatory liabilities 17.0      
San Juan Generating Station | Loss on long-term purchase commitment | PNM        
Public Utilities, Commitments And Contingencies [Line Items]        
Annual funding post-term reclamation trust   $ 2.7    
Annual funding post-term reclamation trust, remainder of year 20.7      
Annual funding post-term reclamation trust, year one 3.9      
Annual funding post-term reclamation trust, year two 4.0      
San Juan Generating Station and Four Corners | Mine Reclamation Trust | PNM        
Public Utilities, Commitments And Contingencies [Line Items]        
Customer reclamation funding cost $ 100.0