Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2020
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	$ 0	$ 0
Interest cost	4,985,000	6,294,000
Expected return on plan assets	(7,363,000)	(8,527,000)
Amortization of net (gain) loss	4,465,000	3,880,000
Amortization of prior service cost	(138,000)	(241,000)
Net Periodic Benefit Cost (Income)	1,949,000	1,406,000
Contributions by employer	0	0
Expected employer contributions, remainder of fiscal year	0
Estimated employer contributions in next fiscal year	0
Expected employer contributions in year 3	4,600,000
Expected employer contributions in year 4	19,100,000
Expected employer contributions in year 5	$ 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
Interest cost	613,000	829,000
Expected return on plan assets	(1,387,000)	(1,318,000)
Amortization of net (gain) loss	87,000	169,000
Amortization of prior service cost	0	(99,000)
Net Periodic Benefit Cost (Income)	(677,000)	(406,000)
Contributions by employer	0	0
Disbursements by employer	1,100,000	800,000
Expected employer disbursements in year 2 through 5	13,500,000
PNM | OPEB Plan | Forecast
Components of Net Periodic Benefit Cost
Disbursements by employer			$ 3,700,000
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	122,000	162,000
Expected return on plan assets	0	0
Amortization of net (gain) loss	101,000	79,000
Amortization of prior service cost	0	0
Net Periodic Benefit Cost (Income)	223,000	241,000
Disbursements by employer	300,000	400,000
Expected employer disbursements in year 2 through 5	5,400,000
Total expected employer disbursements for remainder of fiscal year	1,500,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	544,000	672,000
Expected return on plan assets	(821,000)	(967,000)
Amortization of net (gain) loss	315,000	235,000
Net Periodic Benefit Cost (Income)	38,000	(60,000)
Contributions by employer	0	0
Expected employer contributions in year 4	1,100,000
Expected employer contributions in year 5	2,800,000
Estimated employer contributions remainder of fiscal year through year three	$ 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
Interest cost	93,000	113,000
Expected return on plan assets	(134,000)	(129,000)
Amortization of net (gain) loss	(81,000)	(110,000)
Net Periodic Benefit Cost (Income)	(110,000)	(113,000)
Contributions by employer	0	0
Estimated employer contributions for remainder of fiscal year through year 5	0
TNMP | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	6,000	8,000
Expected return on plan assets	0	0
Amortization of net (gain) loss	6,000	4,000
Net Periodic Benefit Cost (Income)	12,000	12,000
Expected employer disbursements in year 2 through 5	300,000
Total expected employer disbursements for remainder of fiscal year	100,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Disbursements by employer	$ 0	$ 100,000