Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0	$ 0	$ 0
Interest cost	26,908	30,307	28,255
Expected return on plan assets	(33,803)	(35,416)	(39,323)
Amortization of net (gain) loss	16,006	13,820	14,820
Amortization of prior service cost	(965)	(965)	(965)
Net periodic benefit cost	8,146	7,746	2,787
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	96	140	204
Interest cost	4,025	4,346	4,089
Expected return on plan assets	(5,230)	(5,483)	(5,610)
Amortization of net (gain) loss	3,682	1,145	1,966
Amortization of prior service cost	(1,663)	(30)	(642)
Net periodic benefit cost	910	118	7
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	697	812	760
Amortization of net (gain) loss	313	256	325
Amortization of prior service cost	0	0	0
Net periodic benefit cost	1,010	1,068	1,085
Texas-New Mexico Power Company
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Net periodic benefit cost	100	100
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0	0	0
Interest cost	2,887	3,304	3,043
Expected return on plan assets	(3,779)	(3,943)	(4,420)
Amortization of net (gain) loss	923	700	782
Amortization of prior service cost	0	0	0
Net periodic benefit cost	31	61	(595)
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	143	186	247
Interest cost	556	677	608
Expected return on plan assets	(456)	(490)	(520)
Amortization of net (gain) loss	(79)	(40)	0
Amortization of prior service cost	0	0	0
Net periodic benefit cost	164	333	335
Texas-New Mexico Power Company | Executive Retirement Program
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0	0	0
Interest cost	33	40	36
Amortization of net (gain) loss	9	2	5
Amortization of prior service cost	0	0	0
Net periodic benefit cost	$ 42	$ 42	$ 41