Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	4,036,000	4,985,000	12,107,000	14,956,000
Expected return on plan assets	(7,133,000)	(7,363,000)	(21,398,000)	(22,089,000)
Amortization of net loss	4,541,000	4,465,000	13,624,000	13,395,000
Amortization of prior service cost	0	(138,000)	0	(415,000)
Net Periodic Benefit Cost (Income)	1,444,000	1,949,000	4,333,000	5,847,000
Contributions by employer			0	0
Expected employer contributions, remainder of fiscal year	0		$ 0
PNM | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			2.90%
PNM | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	5,000	9,000	$ 18,000	29,000
Interest cost	476,000	613,000	1,430,000	1,840,000
Expected return on plan assets	(1,042,000)	(1,387,000)	(3,125,000)	(4,161,000)
Amortization of net loss	0	87,000	0	261,000
Amortization of prior service cost		0	0	0
Net Periodic Benefit Cost (Income)	(561,000)	(678,000)	(1,677,000)	(2,031,000)
Contributions by employer			0	0
Disbursements by employer	1,000,000	1,000,000	2,700,000	3,100,000
Disbursements by employer, remainder of year	3,400,000		3,400,000
Disbursements by employer, after current year	15,100,000		15,100,000
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	90,000	123,000	271,000	367,000
Expected return on plan assets	0	0	0	0
Amortization of net loss	99,000	101,000	296,000	302,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	189,000	224,000	567,000	669,000
Disbursements by employer	300,000	300,000	1,100,000	1,100,000
Disbursements by employer, remainder of year	1,300,000		1,300,000
Disbursements by employer, after current year	6,000,000		6,000,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0	0	0
Interest cost	435,000	545,000	1,306,000	1,633,000
Expected return on plan assets	(795,000)	(821,000)	(2,386,000)	(2,463,000)
Amortization of net loss	311,000	315,000	935,000	944,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(49,000)	39,000	(145,000)	114,000
Contributions by employer			0	0
Estimated employer contributions for remainder of fiscal year through year 5	0		$ 0
TNMP | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate			2.90%
TNMP | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	12,000	12,000	$ 34,000	35,000
Interest cost	77,000	93,000	231,000	280,000
Expected return on plan assets	(101,000)	(136,000)	(304,000)	(404,000)
Amortization of net loss	(81,000)	(81,000)	(242,000)	(243,000)
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	(93,000)	(112,000)	(281,000)	(332,000)
Contributions by employer	0	0	0	0
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	4,000	6,000	13,000	17,000
Expected return on plan assets	0	0	0	0
Amortization of net loss	8,000	6,000	25,000	18,000
Amortization of prior service cost	0	0	0	0
Net Periodic Benefit Cost (Income)	12,000	12,000	38,000	35,000
Disbursements by employer, remainder of year	100,000		100,000
Disbursements by employer, after current year	400,000		400,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Disbursements by employer	$ 0	$ 0	$ 0	$ 0