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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Public Utilities, Inventory [Line Items]      
Inventory $ 71,834 $ 66,502  
Coal      
Public Utilities, Inventory [Line Items]      
Inventory 22,777 16,714  
Materials and supplies      
Public Utilities, Inventory [Line Items]      
Inventory 49,057 49,788  
Public Service Company of New Mexico      
Public Utilities, Inventory [Line Items]      
Inventory 67,097 60,859  
Public Service Company of New Mexico | Coal      
Public Utilities, Inventory [Line Items]      
Inventory 22,777 16,714  
Public Service Company of New Mexico | Materials and supplies      
Public Utilities, Inventory [Line Items]      
Inventory $ 44,320 $ 44,145  
Public Service Company of New Mexico | Electric plant      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 2.40% 2.52% 2.33%
Public Service Company of New Mexico | Common, intangible, and general plant      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 8.18% 8.36% 5.40%
Texas-New Mexico Power Company      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 3.49% 3.57% 3.66%
Inventory $ 4,737 $ 5,643  
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 $ 4,737 $ 5,643