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Commitments and Contingencies - Coal Supply (Details) - USD ($)
 12 Months Ended
Feb. 01, 2016
Jan. 31, 2016
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Sep. 30, 2018
May 22, 2018
Feb. 01, 2018
Jan. 01, 2018
Feb. 01, 2017
Other Commitments [Line Items]                    
Other current assets     $ 44,472,000 $ 54,808,000            
Public Service Company of New Mexico                    
Other Commitments [Line Items]                    
Other current assets     36,561,000 43,516,000            
Public Service Company of New Mexico | Increase in coal mine decommissioning liability                    
Other Commitments [Line Items]                    
Net expense     800,000              
Public Service Company of New Mexico | Increase in coal mine decommissioning liability | Loss on long-term purchase commitment                    
Other Commitments [Line Items]                    
Estimated underpaid surface mining royalties under proposed rate change           $ 2,500,000        
Public Service Company of New Mexico | Loss on long-term purchase commitment | San Juan Generating Station                    
Other Commitments [Line Items]                    
Annual funding post-term reclamation trust     5,500,000 10,000,000.0 $ 5,800,000          
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station                    
Other Commitments [Line Items]                    
Reclamation trust funding, next fiscal year     9,700,000              
Reclamation trust funding, year 2     10,900,000              
Reclamation trust funding, year 3     11,700,000              
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners                    
Other Commitments [Line Items]                    
Funds contributed to mine reclamation trust     2,300,000              
Funds contributed next fiscal year through year five     1,900,000              
Public Service Company of New Mexico | Surface | Loss on long-term purchase commitment                    
Other Commitments [Line Items]                    
Capped amount that can be collected from retail customers for final reclamation       100,000,000.0            
Liability for interim storage costs     70,300,000 70,100,000            
Public Service Company of New Mexico | Surface | Increase in coal mine decommissioning liability                    
Other Commitments [Line Items]                    
Net expense       29,800,000            
Public Service Company of New Mexico | Underground and Surface | Increase in coal mine decommissioning liability                    
Other Commitments [Line Items]                    
Net expense       39,200,000            
Public Service Company of New Mexico | Underground | Loss on long-term purchase commitment                    
Other Commitments [Line Items]                    
Liability for interim storage costs     25,300,000 23,200,000            
Public Service Company of New Mexico | Underground | Increase in coal mine decommissioning liability                    
Other Commitments [Line Items]                    
Net expense       9,400,000            
Public Service Company of New Mexico | San Juan Generating Station | Surface | Loss on long-term purchase commitment                    
Other Commitments [Line Items]                    
Estimate of possible loss     92,600,000              
Public Service Company of New Mexico | San Juan Generating Station | Underground | Loss on long-term purchase commitment                    
Other Commitments [Line Items]                    
Estimate of possible loss     40,000,000.0              
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply                    
Other Commitments [Line Items]                    
Other current assets       $ 26,300,000            
NM Capital | BTMU Term Loan Agreement                    
Other Commitments [Line Items]                    
Long-term debt $ 125,000,000.0               $ 125,000,000.0  
Repurchased face amount             $ 43,000,000.0      
NM Capital | San Juan Generating Station | Coal Supply                    
Other Commitments [Line Items]                    
Funding provided $ 125,000,000.0 $ 125,000,000.0                
Variable interest rate               12.25%   9.25%
Coal mine reclamation bonds to be posted with NMMMD     118,700,000              
Issuance in letters of credit   $ 30,300,000 $ 30,300,000              
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply                    
Other Commitments [Line Items]                    
Long-term debt                 $ 50,100,000  
Repurchased face amount             $ 50,100,000