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Income Taxes - Carryforwards (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Taxes, Impairment of Carryforwards      
State tax credit carryforwards $ (425) $ 425 $ 0
State net operating loss carryforwards 0 0 0
Charitable contribution carryforwards 0 0 0
Compensation expense 96 (99) 410
Income Taxes, Reserve Balances      
State tax credit carryforwards 0 425  
State net operating loss carryforwards 0 0  
Charitable contribution carryforwards 0 0  
Compensation expense 407 311  
Internal Revenue Service (IRS)      
Operating Loss Carryforwards [Line Items]      
Federal net operating loss carryforwards 222,100    
Federal tax credit carryforwards that expire beginning in 2023 121,400    
Public Service Company of New Mexico      
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 61 (100) 298
Income Taxes, Reserve Balances      
State tax credit carryforwards 0 0  
State net operating loss carryforwards 0 0  
Charitable contribution carryforwards 0 0  
Compensation expense 259 198  
Texas-New Mexico Power Company      
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 35 2 $ 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 $ 148 $ 113