Pension and Other Postretirement Benefits - Actuarial (Gain) Loss Results (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rates	$ 27,547	$ 41,849
Claims, contributions, and demographic experience	(1,249)	(334)
Other assumption and experience	0	(2,052)
Actuarial (gain) loss	26,298	39,463
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rates	3,536	6,569
Claims, contributions, and demographic experience	(5,845)	19,562
Assumed participation rate	0	(6,335)
Mortality rate	0	(691)
Medical benefits	1,425	(1,228)
Dental trend assumption	(717)	0
Actuarial (gain) loss	(1,601)	17,877
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rates	3,528	5,055
Claims, contributions, and demographic experience	(517)	(556)
Other assumption and experience	39	(181)
Actuarial (gain) loss	3,050	4,318
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rates	613	1,112
Claims, contributions, and demographic experience	(994)	(102)
Assumed participation rate	0	(1,013)
Mortality rate	0	(93)
Medical benefits	0	0
Dental trend assumption	0	0
Actuarial (gain) loss	$ (381)	$ (96)