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Summary of the Business and Significant Accounting Policies - Inventories/Depreciation and Amortization (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Public Utilities, Inventory [Line Items]      
Inventory $ 77,929 $ 71,834  
Coal      
Public Utilities, Inventory [Line Items]      
Inventory 24,914 22,777  
Materials and supplies      
Public Utilities, Inventory [Line Items]      
Inventory 53,015 49,057  
Public Service Company of New Mexico      
Public Utilities, Inventory [Line Items]      
Inventory 72,225 67,097  
Public Service Company of New Mexico | Coal      
Public Utilities, Inventory [Line Items]      
Inventory 24,914 22,777  
Public Service Company of New Mexico | Materials and supplies      
Public Utilities, Inventory [Line Items]      
Inventory $ 47,311 $ 44,320  
Public Service Company of New Mexico | Electric plant      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 2.47% 2.40% 2.52%
Public Service Company of New Mexico | Common, intangible, and general plant      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 7.91% 8.18% 8.36%
Texas-New Mexico Power Company      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 4.04% 3.49% 3.57%
Inventory $ 5,704 $ 4,737  
Texas-New Mexico Power Company | Coal      
Public Utilities, Inventory [Line Items]      
Inventory 0 0  
Texas-New Mexico Power Company | Materials and supplies      
Public Utilities, Inventory [Line Items]      
Inventory $ 5,704 $ 4,737