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Pension and Other Postretirement Benefits - Actuarial (Gain) Loss Results (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Public Service Company of New Mexico | Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Discount rates $ (34,769) $ 27,547
Claims, contributions, and demographic experience 431 (1,249)
Other assumptions and experience 279 0
Mortality rate (6,966) 0
Actuarial (gain) loss (41,025) 26,298
Public Service Company of New Mexico | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount rates (4,076) 3,536
Claims, contributions, and demographic experience (3,174) (5,845)
Assumed participation rate (4,040) 0
Mortality rate (916) 0
Medical benefits 0 1,425
Dental trend assumption 0 (717)
Actuarial (gain) loss (12,206) (1,601)
Texas-New Mexico Power Company | Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Discount rates (4,278) 3,528
Claims, contributions, and demographic experience (301) (517)
Other assumptions and experience 68 39
Mortality rate (705) 0
Actuarial (gain) loss (5,216) 3,050
Texas-New Mexico Power Company | Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount rates (710) 613
Claims, contributions, and demographic experience 72 (994)
Assumed participation rate (1,461) 0
Mortality rate (114) 0
Medical benefits 0 0
Dental trend assumption 0 0
Actuarial (gain) loss $ (2,213) $ (381)