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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) Recognized (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Public Service Company of New Mexico | Pension Plan      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost $ 0 $ 0 $ 0
Interest cost 24,270 26,908 30,307
Expected return on plan assets (34,686) (33,803) (35,416)
Amortization of net (gain) loss 16,348 16,006 13,820
Amortization of prior service cost (965) (965) (965)
Net periodic benefit cost 4,967 8,146 7,746
Public Service Company of New Mexico | Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 83 96 140
Interest cost 3,439 4,025 4,346
Expected return on plan assets (5,414) (5,230) (5,483)
Amortization of net (gain) loss 2,354 3,682 1,145
Amortization of prior service cost (1,664) (1,663) (30)
Net periodic benefit cost (1,202) 910 118
Public Service Company of New Mexico | Executive Retirement Program      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 0 0 0
Interest cost 622 697 812
Amortization of net (gain) loss 359 313 256
Amortization of prior service cost 0 0 0
Net periodic benefit cost 981 1,010 1,068
Texas-New Mexico Power Company | Pension Plan      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 0 0 0
Interest cost 2,625 2,887 3,304
Expected return on plan assets (3,963) (3,779) (3,943)
Amortization of net (gain) loss 1,088 923 700
Amortization of prior service cost 0 0 0
Net periodic benefit cost (250) 31 61
Texas-New Mexico Power Company | Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 134 143 186
Interest cost 477 556 677
Expected return on plan assets (542) (456) (490)
Amortization of net (gain) loss (227) (79) (40)
Amortization of prior service cost 0 0 0
Net periodic benefit cost (158) 164 333
Texas-New Mexico Power Company | Executive Retirement Program      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 0 0 0
Interest cost 29 33 40
Amortization of net (gain) loss 15 9 2
Amortization of prior service cost 0 0 0
Net periodic benefit cost $ 44 $ 42 $ 42