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Commitments and Contingencies - Coal Supply (Details)
 12 Months Ended
Oct. 05, 2018
T
Jun. 29, 2018
USD ($)
Feb. 01, 2016
USD ($)
Jan. 31, 2016
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Sep. 30, 2018
USD ($)
May 22, 2018
USD ($)
Feb. 01, 2018
Jun. 30, 2017
Feb. 01, 2017
Jul. 19, 2016
USD ($)
Jul. 07, 2016
payment
Mar. 31, 2016
USD ($)
Other Commitments [Line Items]                                
Expected number of additional monitoring wells         38                      
Other current assets         $ 53,725,000 $ 47,358,000                    
Regulatory disallowances and restructuring costs         65,598,000 27,036,000 $ 15,011,000                  
Funds contributed to mine reclamation trust         2,300,000                      
Public Service Company of New Mexico                                
Other Commitments [Line Items]                                
Other current assets         42,433,000 39,904,000                    
Regulatory disallowances and restructuring costs         66,339,000 27,036,000 15,011,000                  
Public Service Company of New Mexico | Increase in coal mine decommissioning liability                                
Other Commitments [Line Items]                                
Net expense         4,500,000                      
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         10,000,000 5,800,000 $ 7,000,000                  
Public Service Company of New Mexico | Mine Reclamation Trust | San Juan Generating Station                                
Other Commitments [Line Items]                                
Reclamation trust funding, next fiscal year         8,900,000                      
Reclamation trust funding, year 2         10,200,000                      
Reclamation trust funding, year 3         10,900,000                      
Public Service Company of New Mexico | Mine Reclamation Trust | Four Corners                                
Other Commitments [Line Items]                                
Reclamation trust funding, next fiscal year         2,300,000                      
Number of annual installments (in payments) | payment                             13  
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                      
Liability for interim storage costs         70,100,000 41,400,000                    
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         23,200,000 14,700,000                    
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]                                
Regulatory disallowances and restructuring costs               $ 16,500,000                
Estimate of possible loss         103,200,000                      
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         39,700,000                      
Public Service Company of New Mexico | San Juan Generating Station | Coal Supply                                
Other Commitments [Line Items]                                
Other current assets         26,300,000 $ 26,300,000                    
Reduction in coal obligation | T 111,668                              
Estimated increase in coal cost                 6.90%     51.00%        
NM Capital | BTMU Term Loan Agreement                                
Other Commitments [Line Items]                                
Long-term debt     $ 125,000,000.0                          
Repurchased face amount                   $ 43,000,000            
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
Cash used to support bank letter or credit arrangement       $ 30,300,000                   $ 30,300,000    
NM Capital | San Juan Coal Company, Westmoreland | Coal Supply                                
Other Commitments [Line Items]                                
Prepayment penalty         $ 0                      
Repurchased face amount                   $ 50,100,000            
Four Corners Coal Supply Arbitration | Public Service Company of New Mexico                                
Other Commitments [Line Items]                                
Payments for legal settlements   $ 4,900,000                            
Four Corners Coal Supply Arbitration, Period March 1, 2018 through June 30, 2018 | Public Service Company of New Mexico                                
Other Commitments [Line Items]                                
Payments for legal settlements   $ 1,400,000