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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Public Utilities, Inventory [Line Items]      
Inventory $ 67,386 $ 63,628  
Coal [Member]      
Public Utilities, Inventory [Line Items]      
Inventory 18,356 17,525  
Materials and supplies [Member]      
Public Utilities, Inventory [Line Items]      
Inventory 49,030 46,103  
Public Service Company of New Mexico [Member]      
Public Utilities, Inventory [Line Items]      
Inventory 60,477 60,859  
Public Service Company of New Mexico [Member] | Coal [Member]      
Public Utilities, Inventory [Line Items]      
Inventory 18,356 17,525  
Public Service Company of New Mexico [Member] | Materials and supplies [Member]      
Public Utilities, Inventory [Line Items]      
Inventory $ 42,121 $ 43,334  
Public Service Company of New Mexico [Member] | Electric plant [Member]      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 2.27% 2.26% 2.27%
Public Service Company of New Mexico [Member] | Common, intangible, and general plant [Member]      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 4.66% 4.64% 4.87%
Texas-New Mexico Power Company [Member]      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used 3.94% 3.59% 3.66%
Inventory $ 6,909 $ 2,769  
Texas-New Mexico Power Company [Member] | Coal [Member]      
Public Utilities, Inventory [Line Items]      
Inventory 0 0  
Texas-New Mexico Power Company [Member] | Materials and supplies [Member]      
Public Utilities, Inventory [Line Items]      
Inventory $ 6,909 $ 2,769