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Commitments and Contingencies - Coal Supply (Details) - USD ($)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2020
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Mar. 31, 2020
Jan. 31, 2016
Public Utilities, Commitments And Contingencies [Line Items]            
Other current assets $ 64,549,000 $ 60,764,000 $ 64,549,000      
PNM            
Public Utilities, Commitments And Contingencies [Line Items]            
Other current assets 51,908,000 50,441,000 51,908,000      
PNM | Loss on long-term purchase commitment | Surface            
Public Utilities, Commitments And Contingencies [Line Items]            
Loss contingency accrual 71,700,000 70,400,000 71,700,000      
PNM | Loss on long-term purchase commitment | Underground            
Public Utilities, Commitments And Contingencies [Line Items]            
Loss contingency accrual 26,100,000 26,600,000 26,100,000      
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface            
Public Utilities, Commitments And Contingencies [Line Items]            
Estimate of possible loss   78,400,000        
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground            
Public Utilities, Commitments And Contingencies [Line Items]            
Estimate of possible loss   35,100,000        
Coal supply | Four Corners            
Public Utilities, Commitments And Contingencies [Line Items]            
Payments for relief from obligations   75,000,000.0        
Coal supply | PNM | San Juan Generating Station            
Public Utilities, Commitments And Contingencies [Line Items]            
Other current assets 26,300,000 24,800,000 26,300,000      
Coal supply | NM Capital | San Juan Generating Station            
Public Utilities, Commitments And Contingencies [Line Items]            
Requirement to post reclamation bonds   118,700,000        
Cash used to support bank letter or credit arrangement   30,300,000       $ 30,300,000
Increase in coal mine decommissioning liability | PNM            
Public Utilities, Commitments And Contingencies [Line Items]            
Regulatory disallowance, additional amount recorded       $ 800,000    
Increase in coal mine decommissioning liability | PNM | Surface            
Public Utilities, Commitments And Contingencies [Line Items]            
Regulatory disallowance, additional amount recorded 3,600,000          
Decrease in coal mine decommissioning liability | PNM | Underground            
Public Utilities, Commitments And Contingencies [Line Items]            
Regulatory disallowance, additional amount recorded $ 100,000          
Decrease in coal mine decommissioning liability | PNM | Four Corners            
Public Utilities, Commitments And Contingencies [Line Items]            
Regulatory disallowance, additional amount recorded     2,500,000      
Mine Reclamation Trust | PNM            
Public Utilities, Commitments And Contingencies [Line Items]            
Required contribution to reclamation trust, current fiscal year   5,500,000        
Reclamation trust funding, year 2   6,200,000        
Reclamation trust funding, year 3   0        
Four Corners CSA | PNM            
Public Utilities, Commitments And Contingencies [Line Items]            
Other commitment, due in first year         $ 2,000,000.0  
Other commitment, due in second year   2,100,000        
Other commitment, due in third year   2,100,000        
Other commitment, due in fourth year   2,100,000        
Other commitment, due in fifth year   $ 2,100,000        
San Juan Generating Station | Loss on long-term purchase commitment | PNM            
Public Utilities, Commitments And Contingencies [Line Items]            
Annual funding post-term reclamation trust     $ 3,200,000