Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	6,294,000	6,068,000	12,587,000	12,135,000
Expected return on plan assets	(8,527,000)	(8,672,000)	(17,051,000)	(17,343,000)
Amortization of net (gain) loss	3,880,000	4,087,000	7,759,000	8,174,000
Amortization of prior service cost	(241,000)	(241,000)	(483,000)	(483,000)
Net Periodic Benefit Cost (Income)	1,406,000	1,242,000	2,812,000	2,483,000
Contributions by employer			0	0
Expected employer contributions for future fiscal years	0		0
Expected employer contributions in year 5	1,300,000		1,300,000
Expected employer contributions in year 6	22,900,000		$ 22,900,000
PNM | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			4.20%
PNM | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			4.60%
PNM | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	13,000	21,000	$ 26,000	41,000
Interest cost	829,000	860,000	1,658,000	1,720,000
Expected return on plan assets	(1,318,000)	(1,353,000)	(2,636,000)	(2,707,000)
Amortization of net (gain) loss	169,000	588,000	338,000	1,177,000
Amortization of prior service cost	(99,000)	(416,000)	(198,000)	(832,000)
Net Periodic Benefit Cost (Income)	(406,000)	(300,000)	(812,000)	(601,000)
Contributions by employer	700,000	0	1,500,000	0
Estimated future employer contributions for fiscal year	3,700,000		3,700,000
Estimated employer contributions in year 2-5	13,700,000		13,700,000
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	162,000	155,000	324,000	311,000
Expected return on plan assets	0	0	0	0
Amortization of net (gain) loss	79,000	90,000	158,000	179,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	241,000	245,000	482,000	490,000
Contributions by employer	400,000	400,000	700,000	900,000
Total expected employer contributions for fiscal year	1,500,000		1,500,000
Estimated employer contributions in year 2-5	5,600,000		5,600,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	672,000	656,000	1,344,000	1,312,000
Expected return on plan assets	(967,000)	(991,000)	(1,934,000)	(1,981,000)
Amortization of net (gain) loss	235,000	272,000	470,000	544,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(60,000)	(63,000)	(120,000)	(125,000)
Contributions by employer			0	0
Estimated employer contributions in year 2-5	0		$ 0
TNMP | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			4.20%
TNMP | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			4.60%
TNMP | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	13,000	33,000	$ 26,000	67,000
Interest cost	113,000	119,000	226,000	238,000
Expected return on plan assets	(129,000)	(135,000)	(258,000)	(271,000)
Amortization of net (gain) loss	(110,000)	(56,000)	(220,000)	(113,000)
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(113,000)	(39,000)	(226,000)	(79,000)
Contributions by employer	0	0	0	300,000
TNMP | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	8,000	7,000	16,000	15,000
Expected return on plan assets	0	0	0	0
Amortization of net (gain) loss	4,000	4,000	8,000	8,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	12,000	11,000	24,000	23,000
Total expected employer contributions for fiscal year	100,000		100,000
Estimated employer contributions in year 2-5	300,000		300,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Contributions by employer	$ 100,000	$ 100,000	$ 100,000	$ 100,000