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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, 2011
Dec. 31, 2010
Dec. 31, 2009
Public Utilities, Inventory [Line Items]      
Inventory, Net 54,231 52,479  
Coal [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 17,175 17,115  
Materials and supplies [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 37,056 35,364  
Public Service Company of New Mexico [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 51,521 49,454  
Public Service Company of New Mexico [Member] | Coal [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 17,175 17,115  
Public Service Company of New Mexico [Member] | Materials and supplies [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 34,346 32,339  
Public Service Company of New Mexico [Member] | Electric plant [Member]
      
Public Utilities, Inventory [Line Items]      
Depreciation average rates used (as a percent) 2.24% 2.24% 2.31%
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) 6.03% 5.63% 4.29%
Texas-New Mexico Power Company [Member]
      
Public Utilities, Inventory [Line Items]      
Inventory, Net 2,710 2,787  
Depreciation average rates used (as a percent) 3.41% 3.39% 3.44%
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 2,710 2,787