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Commitments and Contingencies - Coal Supply (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Aug. 01, 2016
payment
Jan. 31, 2016
USD ($)
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets $ 53,863,000 $ 54,808,000      
PNM          
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets 43,650,000 43,516,000      
PNM | Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory assets 100,000,000        
PNM | Loss on long-term purchase commitment | Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency accrual 70,400,000 70,100,000      
Final reclamation, capped amount to be collected 100,000,000.0 100,000,000      
PNM | Loss on long-term purchase commitment | Underground          
Public Utilities, Commitments And Contingencies [Line Items]          
Loss contingency accrual 23,800,000 23,200,000      
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Estimate of possible loss 102,600,000        
PNM | Loss on long-term purchase commitment | San Juan Generating Station | Underground          
Public Utilities, Commitments And Contingencies [Line Items]          
Estimate of possible loss 40,000,000        
Coal supply | PNM | San Juan Generating Station          
Public Utilities, Commitments And Contingencies [Line Items]          
Other current assets 26,300,000 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 | Underground and Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory disallowance, additional amount recorded   39,200,000      
Increase in coal mine decommissioning liability | PNM | Surface          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory disallowance, additional amount recorded   29,800,000      
Increase in coal mine decommissioning liability | PNM | Underground          
Public Utilities, Commitments And Contingencies [Line Items]          
Regulatory disallowance, additional amount recorded   9,400,000      
Increase in coal mine decommissioning liability | PNM | Loss on long-term purchase commitment          
Public Utilities, Commitments And Contingencies [Line Items]          
Estimated underpaid surface mining royalties under proposed rate change     $ 2,500,000    
Mine Reclamation Trust | PNM          
Public Utilities, Commitments And Contingencies [Line Items]          
Required contribution to Reclamation Trust, current fiscal year 6,500,000        
Reclamation trust funding, year 2 10,200,000        
Reclamation trust funding, year 3 10,900,000        
San Juan Generating Station | Loss on long-term purchase commitment | PNM          
Public Utilities, Commitments And Contingencies [Line Items]          
Annual funding post-term reclamation trust   10,000,000      
Four Corners | Mine Reclamation Trust | PNM          
Public Utilities, Commitments And Contingencies [Line Items]          
Required contribution to Reclamation Trust, current fiscal year   $ 2,300,000      
Reclamation trust funding, year 2 2,300,000        
Reclamation trust funding, year 3 $ 2,300,000        
Number of annual installment payments | payment       13