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JOINTLY OWNED FACILITIES (Tables)
12 Months Ended
Dec. 31, 2012
JOINTLY OWNED FACILITIES [Abstract]  
Jointly Owned Electric Utility Facilities
  2012
   
    Plant in Accumulated Net Plant in  
  % Owned Service Depreciation Service CWIP
               
NPC             
 Navajo Generating Station11.3% $253,711 $149,969 $103,742 $5,601
 Reid Gardner Generating Station No. 432.2%  184,474  100,277  84,197  9,141
 Silverhawk Generating Station75.0%  247,581  53,235  194,346  2,358
    $685,766 $303,481 $382,285 $17,100
SPPC             
 Valmy Generating Station50.0% $336,038 $214,335 $121,703 $10,808

  2011
    Plant in Accumulated Net Plant in  
  % Owned Service Depreciation Service CWIP
               
NPC             
 Navajo Generating Station11.3% $270,448 $148,582 $121,866 $1,117
 Reid Gardner Generating Station No. 432.2%  171,485  97,042  74,443  7,600
 Silverhawk Generating Station75.0%  247,342  50,822  196,520  203
    $689,275 $296,446 $392,829 $8,920
SPPC             
 Valmy Generating Station50.0% $331,753 $215,642 $116,111 $6,682