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Income Taxes - Carryforwards (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2013
Dec. 31, 2012
Operating Loss Carryforwards [Line Items]          
New Mexico corporate tax rate being phased in         7.60%
New Mexico Corporate tax rate, effective by 2018       5.90%  
Regulatory liability   $ (10,109) $ (7,132)    
Income tax expense   (1,259) 712    
Income Taxes, Impairment of Carryforwards          
State tax credit carryforwards $ 0 0 0    
State net operating loss carryforwards 0 819 (311)    
Charitable contribution carryforwards 0 909 0    
Compensation expense 410        
Income Taxes, Reserve Balances          
State tax credit carryforwards 0 2,487      
State net operating loss carryforwards 0 1,131      
Charitable contribution carryforwards 0 952      
Compensation expense 410        
Internal Revenue Service (IRS)          
Operating Loss Carryforwards [Line Items]          
Federal net operating loss carryforwards 474,600        
Federal tax credit carryforwards that expire beginning in 2023 76,500        
Public Service Company of New Mexico          
Operating Loss Carryforwards [Line Items]          
Regulatory liability   (10,109) (7,132)    
Income tax expense   (1,179) 804    
Income Taxes, Impairment of Carryforwards          
State tax credit carryforwards 0 0 0    
State net operating loss carryforwards 0 627 (213)    
Charitable contribution carryforwards 0 0 0    
Compensation expense 298        
Income Taxes, Reserve Balances          
State tax credit carryforwards 0 0      
State net operating loss carryforwards 0 839      
Charitable contribution carryforwards 0 0      
Compensation expense 298        
Texas-New Mexico Power Company          
Operating Loss Carryforwards [Line Items]          
Regulatory liability   0 0    
Income tax expense   0 0    
Income Taxes, Impairment of Carryforwards          
State tax credit carryforwards 0 0 0    
State net operating loss carryforwards 0 0 0    
Charitable contribution carryforwards 0 0 $ 0    
Compensation expense 111        
Income Taxes, Reserve Balances          
State tax credit carryforwards 0 0      
State net operating loss carryforwards 0 0      
Charitable contribution carryforwards 0 $ 0      
Compensation expense 111        
2015 Electric Rate Case | Public Service Company of New Mexico          
Income Taxes, Reserve Balances          
Approval to recover impairment of net operating loss carryforward 2,100        
Recovery period of regulatory asset   2 years      
2016 Electric Rate Case | Public Service Company of New Mexico          
Income Taxes, Reserve Balances          
Approval to recover impairment of net operating loss carryforward $ 3,300