Pension and Other Postretirement Benefit Plans (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Other (deductions)	$ 2,624,000	$ 6,709,000		$ 9,867,000	$ 17,372,000
Other income	3,735,000	6,275,000		12,000,000	14,626,000
PNM
Defined Benefit Plan Disclosure [Line Items]
Other (deductions)	2,085,000	4,964,000		7,314,000	14,490,000
Other income	2,137,000	3,762,000		6,821,000	10,270,000
PNM | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0		0	0
Interest cost	6,068,000	6,727,000		18,203,000	20,181,000
Expected return on plan assets	(8,672,000)	(8,451,000)		(26,014,000)	(25,352,000)
Amortization of net (gain) loss	4,087,000	4,001,000		12,261,000	12,004,000
Amortization of prior service cost	(241,000)	(241,000)		(724,000)	(724,000)
Net Periodic Benefit Cost	1,242,000	2,036,000		3,726,000	6,109,000
Contributions by employer				0	0
Total expected employer contributions for future fiscal years	0			0
Expected employer contributions in year 5	5,500,000			$ 5,500,000
PNM | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate				4.00%
PNM | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate				5.10%
PNM | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	21,000	24,000		$ 62,000	72,000
Interest cost	860,000	1,006,000		2,579,000	3,019,000
Expected return on plan assets	(1,353,000)	(1,308,000)		(4,061,000)	(3,923,000)
Amortization of net (gain) loss	588,000	921,000		1,765,000	2,762,000
Amortization of prior service cost	(416,000)	(416,000)		(1,248,000)	(1,248,000)
Net Periodic Benefit Cost	(300,000)	227,000		(903,000)	682,000
Contributions by employer				0	0
PNM | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0		0	0
Interest cost	155,000	174,000		467,000	523,000
Expected return on plan assets	0	0		0	0
Amortization of net (gain) loss	90,000	78,000		269,000	235,000
Amortization of prior service cost	0	0		0	0
Net Periodic Benefit Cost	245,000	252,000		736,000	758,000
Contributions by employer	400,000	400,000		1,300,000	1,200,000
Total expected employer contributions for fiscal year	1,600,000			1,600,000
Estimated employer contributions in Year 2-5	5,700,000			5,700,000
TNMP
Defined Benefit Plan Disclosure [Line Items]
Other (deductions)	149,000	1,030,000		1,209,000	1,229,000
Other income	1,300,000	2,258,000		4,276,000	3,621,000
TNMP | Pension Plan
Components of Net Periodic Benefit Cost
Service cost	0	0		0	0
Interest cost	656,000	722,000		1,969,000	2,165,000
Expected return on plan assets	(991,000)	(945,000)		(2,972,000)	(2,834,000)
Amortization of net (gain) loss	272,000	231,000		816,000	692,000
Amortization of prior service cost	0	0		0	0
Net Periodic Benefit Cost	(63,000)	8,000		(187,000)	23,000
Contributions by employer				0	0
Estimated employer contributions in Year 2-5	0			$ 0
TNMP | Pension Plan | Minimum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate				4.00%
TNMP | Pension Plan | Maximum
Components of Net Periodic Benefit Cost
Assumptions used calculating net periodic benefit cost, discount rate				5.10%
TNMP | OPEB Plan
Components of Net Periodic Benefit Cost
Service cost	33,000	36,000		$ 100,000	107,000
Interest cost	119,000	139,000		358,000	417,000
Expected return on plan assets	(135,000)	(114,000)		(406,000)	(342,000)
Amortization of net (gain) loss	(56,000)	(20,000)		(170,000)	(60,000)
Amortization of prior service cost	0	0		0	0
Net Periodic Benefit Cost	(39,000)	41,000		(118,000)	122,000
Contributions by employer	0	0		300,000	700,000
Estimated employer contributions in Year 2-5	1,400,000			1,400,000
TNMP | Executive Retirement Program
Components of Net Periodic Benefit Cost
Service cost	0	0		0	0
Interest cost	7,000	8,000		22,000	25,000
Expected return on plan assets	0	0		0	0
Amortization of net (gain) loss	4,000	2,000		11,000	7,000
Amortization of prior service cost	0	0		0	0
Net Periodic Benefit Cost	11,000	10,000		33,000	32,000
Total expected employer contributions for fiscal year	100,000			100,000
Estimated employer contributions in Year 2-5	400,000			400,000
TNMP | Executive Retirement Program | Maximum
Components of Net Periodic Benefit Cost
Contributions by employer	100,000	100,000		100,000	100,000
Accounting Standards Update 2017-07
Defined Benefit Plan Disclosure [Line Items]
Other (deductions)		$ 2,100,000			$ 6,400,000
Accounting Standards Update 2017-07 | PNM
Defined Benefit Plan Disclosure [Line Items]
Other (deductions)	1,100,000			3,200,000
Non-service cost deferred as regulatory assets	100,000			300,000
Accounting Standards Update 2017-07 | TNMP
Defined Benefit Plan Disclosure [Line Items]
Other income	100,000			300,000
Non-service cost deferred as regulatory liability	$ 100,000			$ 100,000
Forecast | TNMP | OPEB Plan
Components of Net Periodic Benefit Cost
Contributions by employer			$ 0