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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Public Service Company of New Mexico | Pension Plan      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost $ 0 $ 0 $ 0
Interest cost 25,175 24,270 26,908
Expected return on plan assets (34,103) (34,686) (33,803)
Amortization of net (gain) loss 15,518 16,348 16,006
Amortization of prior service cost (965) (965) (965)
Net periodic benefit cost 5,625 4,967 8,146
Public Service Company of New Mexico | Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 53 83 96
Interest cost 3,316 3,439 4,025
Expected return on plan assets (5,278) (5,414) (5,230)
Amortization of net (gain) loss 675 2,354 3,682
Amortization of prior service cost (397) (1,664) (1,663)
Net periodic benefit cost (1,631) (1,202) 910
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 651 622 697
Amortization of net (gain) loss 318 359 313
Amortization of prior service cost 0 0 0
Net periodic benefit cost 969 981 1,010
Texas-New Mexico Power Company | Pension Plan      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 0 0 0
Interest cost 2,686 2,625 2,887
Expected return on plan assets (3,868) (3,963) (3,779)
Amortization of net (gain) loss 941 1,088 923
Amortization of prior service cost 0 0 0
Net periodic benefit cost (241) (250) 31
Texas-New Mexico Power Company | Other Postretirement Benefits      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 50 134 143
Interest cost 451 477 556
Expected return on plan assets (517) (542) (456)
Amortization of net (gain) loss (444) (227) (79)
Amortization of prior service cost 0 0 0
Net periodic benefit cost (460) (158) 164
Texas-New Mexico Power Company | Executive Retirement Program      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 0 0 0
Interest cost 30 29 33
Amortization of net (gain) loss 15 15 9
Amortization of prior service cost 0 0 0
Net periodic benefit cost $ 45 $ 44 $ 42