Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	4,985,000	6,294,000	9,971,000	12,587,000
Expected return on plan assets	(7,363,000)	(8,527,000)	(14,726,000)	(17,051,000)
Amortization of net (gain) loss	4,465,000	3,880,000	8,930,000	7,759,000
Amortization of prior service cost	(138,000)	(241,000)	(277,000)	(483,000)
Net Periodic Benefit Cost (Income)	1,949,000	1,406,000	3,898,000	2,812,000
Contributions by employer			0	0
Expected employer contributions, remainder of fiscal year	0		0
Estimated employer contributions in next fiscal year	0		0
Expected employer contributions in year 3	4,600,000		4,600,000
Expected employer contributions in year 4	19,100,000		19,100,000
Expected employer contributions in year 5	19,000,000.0		$ 19,000,000.0
PNM | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			3.40%
PNM | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			3.50%
PNM | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	10,000	13,000	$ 20,000	26,000
Interest cost	613,000	829,000	1,227,000	1,658,000
Expected return on plan assets	(1,387,000)	(1,318,000)	(2,774,000)	(2,636,000)
Amortization of net (gain) loss	87,000	169,000	174,000	338,000
Amortization of prior service cost	0	(99,000)	0	(198,000)
Net Periodic Benefit Cost (Income)	(677,000)	(406,000)	(1,353,000)	(812,000)
Contributions by employer			0	0
Disbursements by employer	1,000,000.0	700,000	2,100,000	1,500,000
Expected employer disbursements in year 2 through 5	13,500,000		13,500,000
Total expected employer disbursements for remainder of fiscal year	3,700,000		3,700,000
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	122,000	162,000	244,000	324,000
Expected return on plan assets	0	0	0	0
Amortization of net (gain) loss	101,000	79,000	201,000	158,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	223,000	241,000	445,000	482,000
Disbursements by employer	300,000	400,000	800,000	700,000
Expected employer disbursements in year 2 through 5	5,400,000		5,400,000
Total expected employer disbursements for remainder of fiscal year	1,500,000		1,500,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	544,000	672,000	1,088,000	1,344,000
Expected return on plan assets	(821,000)	(967,000)	(1,642,000)	(1,934,000)
Amortization of net (gain) loss	315,000	235,000	629,000	470,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	38,000	(60,000)	75,000	(120,000)
Contributions by employer			0	0
Expected employer contributions in year 4	1,100,000		1,100,000
Expected employer contributions in year 5	2,800,000		2,800,000
Estimated employer contributions remainder of fiscal year through year three	0		$ 0
TNMP | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			3.40%
TNMP | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			3.50%
TNMP | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	12,000	13,000	$ 23,000	26,000
Interest cost	93,000	113,000	187,000	226,000
Expected return on plan assets	(135,000)	(129,000)	(268,000)	(258,000)
Amortization of net (gain) loss	(80,000)	(110,000)	(162,000)	(220,000)
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(110,000)	(113,000)	(220,000)	(226,000)
Disbursements by employer	200,000	100,000	300,000	200,000
Estimated employer contributions for remainder of fiscal year through year 5	0		0
TNMP | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	6,000	8,000	11,000	16,000
Expected return on plan assets	0	0	0	0
Amortization of net (gain) loss	6,000	4,000	12,000	8,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	12,000	12,000	23,000	24,000
Expected employer disbursements in year 2 through 5	300,000		300,000
Total expected employer disbursements for remainder of fiscal year	100,000		100,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Disbursements by employer	$ 0	$ 0	$ 0	$ 0