Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	$ 0	$ 0
Interest cost	6,294,000	6,068,000
Expected return on plan assets	(8,527,000)	(8,672,000)
Amortization of net (gain) loss	3,880,000	4,087,000
Amortization of prior service cost	(241,000)	(241,000)
Net Periodic Benefit Cost	1,406,000	1,242,000
Contributions by employer	0	0
Expected employer contributions for future fiscal years	0
Expected employer contributions in year 5	1,300,000
Expected employer contributions in year 6	$ 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
Interest cost	829,000	860,000
Expected return on plan assets	(1,318,000)	(1,353,000)
Amortization of net (gain) loss	169,000	588,000
Amortization of prior service cost	(99,000)	(416,000)
Net Periodic Benefit Cost	(406,000)	(300,000)
Contributions by employer	0	0
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	162,000	155,000
Expected return on plan assets	0	0
Amortization of net (gain) loss	79,000	90,000
Amortization of prior service cost	0	0
Net Periodic Benefit Cost	241,000	245,000
Contributions by employer	400,000	500,000
Total expected employer contributions for fiscal year	1,500,000
Estimated employer contributions in Year 2-5	5,600,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	672,000	656,000
Expected return on plan assets	(967,000)	(991,000)
Amortization of net (gain) loss	235,000	272,000
Amortization of prior service cost	0	0
Net Periodic Benefit Cost	(60,000)	(63,000)
Contributions by employer	0	0
Estimated employer contributions in Year 2-5	$ 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
Interest cost	113,000	119,000
Expected return on plan assets	(129,000)	(135,000)
Amortization of net (gain) loss	(110,000)	(56,000)
Amortization of prior service cost	0	0
Net Periodic Benefit Cost	(113,000)	(39,000)
Contributions by employer	0	300,000
TNMP | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0
Interest cost	8,000	7,000
Expected return on plan assets	0	0
Amortization of net (gain) loss	4,000	4,000
Amortization of prior service cost	0	0
Net Periodic Benefit Cost	12,000	11,000
Total expected employer contributions for fiscal year	100,000
Estimated employer contributions in Year 2-5	300,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Contributions by employer	$ 100,000	$ 100,000