Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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	14,956,000	18,881,000
Expected return on plan assets	(7,363,000)	(8,527,000)	(22,089,000)	(25,577,000)
Amortization of net loss	4,465,000	3,880,000	13,395,000	11,639,000
Amortization of prior service cost	(138,000)	(241,000)	(415,000)	(724,000)
Net Periodic Benefit Cost (Income)	1,949,000	1,406,000	5,847,000	4,219,000
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	9,000	13,000	$ 29,000	39,000
Interest cost	613,000	829,000	1,840,000	2,487,000
Expected return on plan assets	(1,387,000)	(1,320,000)	(4,161,000)	(3,956,000)
Amortization of net loss	87,000	169,000	261,000	507,000
Amortization of prior service cost	0	(99,000)	0	(297,000)
Net Periodic Benefit Cost (Income)	(678,000)	(408,000)	(2,031,000)	(1,220,000)
Contributions by employer	0		0	0
Disbursements by employer	1,000,000.0	900,000	3,100,000	2,400,000
Total expected employer disbursements for remainder of fiscal year	3,700,000		3,700,000
Expected employer disbursements in year 2 through 5	13,500,000		13,500,000
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	123,000	163,000	367,000	487,000
Expected return on plan assets	0	0	0	0
Amortization of net loss	101,000	79,000	302,000	237,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	224,000	242,000	669,000	724,000
Disbursements by employer	300,000	400,000	1,100,000	1,100,000
Total expected employer disbursements for remainder of fiscal year	1,500,000		1,500,000
Expected employer disbursements in year 2 through 5	5,400,000		5,400,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	545,000	672,000	1,633,000	2,015,000
Expected return on plan assets	(821,000)	(967,000)	(2,463,000)	(2,901,000)
Amortization of net loss	315,000	235,000	944,000	706,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	39,000	(60,000)	114,000	(180,000)
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	12,000	$ 35,000	39,000
Interest cost	93,000	113,000	280,000	338,000
Expected return on plan assets	(136,000)	(129,000)	(404,000)	(387,000)
Amortization of net loss	(81,000)	(111,000)	(243,000)	(332,000)
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(112,000)	(115,000)	(332,000)	(342,000)
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	7,000	17,000	24,000
Expected return on plan assets	0	0	0	0
Amortization of net loss	6,000	4,000	18,000	11,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	12,000	11,000	35,000	35,000
Total expected employer disbursements for remainder of fiscal year	100,000		100,000
Expected employer disbursements in year 2 through 5	300,000		300,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Disbursements by employer	$ 0	$ 0	$ 0	$ 0