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Summary of Business and Significant Accounting Policies (Inventories/Depreciation and Amortization) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Public Utilities, Inventory [Line Items]      
Inventory, Net 59,643 54,231  
Coal [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 19,231 17,175  
Materials and supplies [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 40,412 37,056  
Public Service Company of New Mexico [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 56,790 51,521  
Public Service Company of New Mexico [Member] | Coal [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 19,231 17,175  
Public Service Company of New Mexico [Member] | Materials and supplies [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 37,559 34,346  
Public Service Company of New Mexico [Member] | Electric plant [Member]
      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used (as a percent) 2.25% 2.24% 2.24%
Public Service Company of New Mexico [Member] | Common, intangible, and general plant [Member]
      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used (as a percent) 5.35% 6.03% 5.63%
Texas-New Mexico Power Company [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 2,853 2,710  
Depreciation average rates used (as a percent) 3.56% 3.41% 3.39%
Texas-New Mexico Power Company [Member] | Coal [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 0 0  
Texas-New Mexico Power Company [Member] | Materials and supplies [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 2,853 2,710