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Pension and Other Postretirement Benefit Plans Components of Net Periodic Benefit Cost (Income) Recognized (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Public Service Company of New Mexico [Member] | Pension Plan [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost $ 0 $ 0 $ 0
Interest cost 32,232 32,804 34,073
Expected return on plan assets (41,301) (37,075) (37,354)
Amortization of net (gain) loss 10,516 9,209 6,450
Amortization of prior service cost (credit) 317 317 317
Net periodic benefit cost 1,764 5,255 3,486
Public Service Company of New Mexico [Member] | Executive Retirement Program [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost 0 0 0
Interest cost 876 930 1,053
Amortization of net (gain) loss 83 93 71
Amortization of prior service cost (credit) 0 0 0
Net periodic benefit cost 959 1,023 1,124
Public Service Company of New Mexico [Member] | Other Postretirement Benefits [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost 217 259 419
Interest cost 5,293 5,378 7,650
Expected return on plan assets (4,901) (5,388) (5,572)
Amortization of net (gain) loss 3,888 3,205 5,489
Amortization of prior service cost (credit) (1,343) (2,648) (4,143)
Net periodic benefit cost 3,154 806 3,843
Texas-New Mexico Power Company [Member] | Pension Plan [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost 0 0 0
Interest cost 3,635 3,800 4,126
Expected return on plan assets (5,324) (5,470) (5,794)
Amortization of net (gain) loss 462 346 0
Amortization of prior service cost (credit) 0 0 0
Net periodic benefit cost (1,227) (1,324) (1,668)
Texas-New Mexico Power Company [Member] | Executive Retirement Program [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost 0 0 0
Interest cost 45 46 52
Amortization of net (gain) loss 0 0 (4)
Amortization of prior service cost (credit) 0 0 0
Net periodic benefit cost 45 46 48
Texas-New Mexico Power Company [Member] | Other Postretirement Benefits [Member]
      
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]      
Service cost 244 306 289
Interest cost 624 654 711
Expected return on plan assets (516) (533) (514)
Amortization of net (gain) loss (209) (193) (195)
Amortization of prior service cost (credit) 57 60 60
Net periodic benefit cost $ 200 $ 294 $ 351