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Commitments and Contingencies - Coal Supply (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Jan. 31, 2016
Other Commitments [Line Items]        
Coal requirement, minimum required payment, year one $ 39.9      
Coal requirement, minimum required payment, year two 81.8      
Public Utilities, Coal Requirement, Coal requirement, minimum required payment, year three 83.8      
Coal requirement, minimum required payment, year four 111.6      
PNM | Loss on long-term purchase commitment | San Juan Generating Station        
Other Commitments [Line Items]        
Annual funding post-term reclamation trust 2.7 $ 10.0 $ 5.2  
PNM | Mine Reclamation Trust | Four Corners        
Other Commitments [Line Items]        
Funds contributed to mine reclamation trust 0.2 2.4 $ 2.2  
Funds contributed next fiscal year through year five 2.1      
PNM | Mine Reclamation Trust | San Juan Generating Station And Four Corners        
Other Commitments [Line Items]        
Customer reclamation funding cost, maximum 100.0      
PNM | Surface | Loss on long-term purchase commitment        
Other Commitments [Line Items]        
Liability for interim storage costs 50.0 62.6    
PNM | Underground | Loss on long-term purchase commitment        
Other Commitments [Line Items]        
Liability for interim storage costs 26.2 $ 28.2    
PNM | San Juan Generating Station | Surface | Loss on long-term purchase commitment        
Other Commitments [Line Items]        
Estimate of possible loss 53.6      
PNM | San Juan Generating Station | Underground | Loss on long-term purchase commitment        
Other Commitments [Line Items]        
Estimate of possible loss 30.4      
NM Capital | San Juan Generating Station | Coal Supply        
Other Commitments [Line Items]        
Coal mine reclamation bonds to be posted with NMMMD 118.7      
Issuance in letters of credit $ 30.3     $ 30.3