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Pension and Other Postretirement Benefit Plans Assumptions Used (Details)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]      
Expected return on plan assets 4.00%    
Texas-New Mexico Power Company [Member] | Executive Retirement Program [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.19% 5.69% 5.50%
Discount rate for determining net periodic benefit cost (income) 5.69% 5.50% 6.31%
Texas-New Mexico Power Company [Member] | Other Postretirement Benefits [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.26% 5.70% 5.59%
Discount rate for determining net periodic benefit cost (income) 5.70% 5.59% 6.42%
Expected return on plan assets 6.50% 6.30% 6.70%
Texas-New Mexico Power Company [Member] | Pension Plan [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.19% 5.69% 5.50%
Discount rate for determining net periodic benefit cost (income) 5.69% 5.50% 6.31%
Expected return on plan assets 8.25% 8.50% 8.75%
Public Service Company of New Mexico [Member] | Executive Retirement Program [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.30% 5.67% 5.72%
Discount rate for determining net periodic benefit cost (income) 5.67% 5.72% 6.47%
Public Service Company of New Mexico [Member] | Other Postretirement Benefits [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.26% 5.70% 5.59%
Discount rate for determining net periodic benefit cost (income) 5.70% 5.59% 6.42%
Expected return on plan assets 8.50% 8.50% 8.75%
Public Service Company of New Mexico [Member] | Pension Plan [Member]
      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate for determining December 31 4.30% 5.67% 5.72%
Discount rate for determining net periodic benefit cost (income) 5.67% 5.72% 6.47%
Expected return on plan assets 8.25% 8.50% 8.75%