Pension and Other Postretirement Benefits - APBO, PBO, Fair Value of Plan Assets, and Funded Status of the Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Non-current liability	$ 94,003	$ 125,844
Public Service Company of New Mexico
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Non-current liability	86,124	114,427
Public Service Company of New Mexico | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	621,751	597,900
Service cost	0	0	$ 0
Interest cost	26,908	30,307	28,255
Actuarial (gain) loss	26,298	39,463
Benefits paid	(50,974)	(45,919)
Balance at end of year	623,983	621,751	597,900
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	543,601	557,923
Actual return on plan assets	69,389	31,597
Employer contributions	0	0
Fair value of plan assets at end of year	562,016	543,601	557,923
Funded status – asset (liability) for pension benefits	(61,967)	(78,150)
Public Service Company of New Mexico | Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	94,269	84,674
Service cost	96	140	204
Interest cost	4,025	4,346	4,089
Participant contributions	3,069	2,690
Actuarial (gain) loss	(1,601)	17,877
Benefits paid	(9,961)	(11,734)
Plan design changes	0	(3,724)
Balance at end of year	89,897	94,269	84,674
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	72,694	72,952
Actual return on plan assets	14,222	5,923
Employer contributions	332	2,863
Fair value of plan assets at end of year	80,356	72,694	72,952
Funded status – asset (liability) for pension benefits	(9,541)	(21,575)
Public Service Company of New Mexico | Executive Retirement Program
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	16,212	16,105
Service cost	0	0	0
Interest cost	697	812	760
Actuarial (gain) loss	674	768
Benefits paid	(1,466)	(1,473)
Balance at end of year	16,117	16,212	16,105
Less current liability	1,501	1,510
Non-current liability	14,616	14,702
Texas-New Mexico Power Company
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Non-current liability	7,879	11,417
Texas-New Mexico Power Company | Pension Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	67,061	64,198
Service cost	0	0	0
Interest cost	2,887	3,304	3,043
Actuarial (gain) loss	3,050	4,318
Benefits paid	(4,575)	(4,759)
Balance at end of year	68,423	67,061	64,198
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	60,624	62,082
Actual return on plan assets	7,450	3,301
Employer contributions	0	0
Fair value of plan assets at end of year	63,499	60,624	62,082
Funded status – asset (liability) for pension benefits	(4,924)	(6,437)
Texas-New Mexico Power Company | Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	12,830	13,106
Service cost	143	186	247
Interest cost	556	677	608
Participant contributions	379	520
Actuarial (gain) loss	(381)	(96)
Benefits paid	(1,248)	(1,563)
Plan design changes	0	0
Balance at end of year	12,279	12,830	13,106
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	8,544	9,111
Actual return on plan assets	1,642	476
Employer contributions	685	0
Fair value of plan assets at end of year	10,002	8,544	9,111
Funded status – asset (liability) for pension benefits	(2,277)	(4,286)
Texas-New Mexico Power Company | Executive Retirement Program
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of year	787	794
Service cost	0	0	0
Interest cost	33	40	36
Actuarial (gain) loss	44	47
Benefits paid	(93)	(94)
Balance at end of year	771	787	$ 794
Less current liability	93	93
Non-current liability	$ 678	$ 694