Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 357,649	$ 330,228	$ 691,271	$ 679,872
Other operating expenses	122,614	273,996	240,815	399,935
Depreciation and amortization	70,022	66,065	138,995	131,421
Operating income (loss)	71,150	(93,615)	118,888	(56,892)
Interest income	3,071	3,460	6,494	7,048
Other income (deductions)	22,703	4,832	(11,303)	19,039
Interest charges	(31,088)	(29,791)	(61,522)	(61,425)
Earnings (Loss) before Income Taxes	65,836	(115,114)	52,557	(92,230)
Income taxes (benefit)	4,275	(42,831)	2,395	(41,608)
Net Earnings (Loss)	61,561	(72,283)	50,162	(50,622)
Valencia non-controlling interest	(3,940)	(3,499)	(7,669)	(6,328)
Subsidiary preferred stock dividends	(132)	(132)	(264)	(264)
Net Earnings (Loss) Available for PNM Common Stock	57,489	(75,914)	42,229	(57,214)
Total Assets	7,574,467	7,048,667	7,574,467	7,048,667	$ 7,298,774
Goodwill	278,297	278,297	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	101,557	255,519	200,113	361,981
Depreciation and amortization	41,763	39,811	83,212	79,036
Operating income (loss)	49,584	(115,977)	83,188	(91,685)
Interest income	3,147	3,530	6,643	7,187
Other income (deductions)	20,954	4,179	(13,072)	18,536
Interest charges	(19,178)	(18,526)	(36,807)	(36,886)
Earnings (Loss) before Income Taxes	54,507	(126,794)	39,952	(102,848)
Income taxes (benefit)	4,895	(43,481)	2,536	(41,508)
Net Earnings (Loss)	49,612	(83,313)	37,416	(61,340)
Valencia non-controlling interest	(3,940)	(3,499)	(7,669)	(6,328)
Subsidiary preferred stock dividends	(132)	(132)	(264)	(264)
Net Earnings (Loss) Available for PNM Common Stock	45,540	(86,944)	29,483	(67,932)
Total Assets	5,377,422	5,105,090	5,377,422	5,105,090
Goodwill	51,632	51,632	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	25,339	24,013	50,461	49,253
Depreciation and amortization	22,368	20,502	44,204	40,716
Operating income (loss)	23,175	22,578	37,520	35,163
Interest income	0	0	0	0
Other income (deductions)	1,963	731	2,524	1,317
Interest charges	(7,361)	(6,560)	(14,533)	(15,361)
Earnings (Loss) before Income Taxes	17,777	16,749	25,511	21,119
Income taxes (benefit)	1,603	1,482	2,245	1,754
Net Earnings (Loss)	16,174	15,267	23,266	19,365
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings (Loss) Available for PNM Common Stock	16,174	15,267	23,266	19,365
Total Assets	1,985,721	1,768,831	1,985,721	1,768,831
Goodwill	226,665	226,665	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	(4,282)	(5,536)	(9,759)	(11,299)
Depreciation and amortization	5,891	5,752	11,579	11,669
Operating income (loss)	(1,609)	(216)	(1,820)	(370)
Interest income	(76)	(70)	(149)	(139)
Other income (deductions)	(214)	(78)	(755)	(814)
Interest charges	(4,549)	(4,705)	(10,182)	(9,178)
Earnings (Loss) before Income Taxes	(6,448)	(5,069)	(12,906)	(10,501)
Income taxes (benefit)	(2,223)	(832)	(2,386)	(1,854)
Net Earnings (Loss)	(4,225)	(4,237)	(10,520)	(8,647)
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings (Loss) Available for PNM Common Stock	(4,225)	(4,237)	(10,520)	(8,647)
Total Assets	211,324	174,746	211,324	174,746
Goodwill	0	0	0	0
Cost of energy
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	357,649	330,228	691,271	679,872
Cost of energy	93,863	83,782	192,573	205,408
Utility margin	263,786	246,446	498,698	474,464
Cost of energy | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	260,788	238,219	508,921	507,536
Cost of energy	67,884	58,866	142,408	158,204
Utility margin	192,904	179,353	366,513	349,332
Cost of energy | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	96,861	92,009	182,350	172,336
Cost of energy	25,979	24,916	50,165	47,204
Utility margin	70,882	67,093	132,185	125,132
Cost of energy | Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	0	0	0	0
Cost of energy	0	0	0	0
Utility margin	$ 0	$ 0	$ 0	$ 0