Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 472,465	$ 433,586	$ 1,163,736	$ 1,113,458
Other operating expenses	122,269	115,960	363,084	515,894
Depreciation and amortization	68,400	68,350	207,395	199,771
Operating income	147,805	140,540	266,693	83,648
Interest income	3,180	3,440	9,674	10,489
Other income (deductions)	14,062	2,330	2,759	21,369
Interest charges	(27,263)	(30,359)	(88,785)	(91,785)
Earnings before Income Taxes	137,784	115,951	190,341	23,721
Income taxes (benefit)	12,331	9,188	14,726	(32,420)
Net Earnings	125,453	106,763	175,615	56,141
Valencia non-controlling interest	(3,553)	(3,860)	(11,222)	(10,188)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	121,768	102,771	163,997	45,557
Total Assets	7,742,112	7,200,932	7,742,112	7,200,932	$ 7,298,774
Goodwill	278,297	278,297	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	99,171	97,200	299,284	459,180
Depreciation and amortization	40,509	40,545	123,721	119,581
Operating income	116,540	108,453	199,728	16,768
Interest income	3,239	3,515	9,882	10,701
Other income (deductions)	11,563	770	(1,509)	19,307
Interest charges	(14,747)	(18,492)	(51,554)	(55,379)
Earnings before Income Taxes	116,595	94,246	156,547	(8,603)
Income taxes (benefit)	13,591	9,525	16,127	(31,984)
Net Earnings	103,004	84,721	140,420	23,381
Valencia non-controlling interest	(3,553)	(3,860)	(11,222)	(10,188)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	99,319	80,729	128,802	12,797
Total Assets	5,449,238	5,176,831	5,449,238	5,176,831
Goodwill	51,632	51,632	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	27,983	24,051	78,444	73,303
Depreciation and amortization	22,492	22,005	66,696	62,722
Operating income	31,779	32,596	69,299	67,759
Interest income	0	0	0	0
Other income (deductions)	2,286	1,766	4,810	3,083
Interest charges	(7,942)	(7,047)	(22,475)	(22,408)
Earnings before Income Taxes	26,123	27,315	51,634	48,434
Income taxes (benefit)	2,202	2,228	4,447	3,982
Net Earnings	23,921	25,087	47,187	44,452
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	23,921	25,087	47,187	44,452
Total Assets	2,075,291	1,832,066	2,075,291	1,832,066
Goodwill	226,665	226,665	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	(4,885)	(5,291)	(14,644)	(16,589)
Depreciation and amortization	5,399	5,800	16,978	17,468
Operating income	(514)	(509)	(2,334)	(879)
Interest income	(59)	(75)	(208)	(212)
Other income (deductions)	213	(206)	(542)	(1,021)
Interest charges	(4,574)	(4,820)	(14,756)	(13,998)
Earnings before Income Taxes	(4,934)	(5,610)	(17,840)	(16,110)
Income taxes (benefit)	(3,462)	(2,565)	(5,848)	(4,418)
Net Earnings	(1,472)	(3,045)	(11,992)	(11,692)
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	(1,472)	(3,045)	(11,992)	(11,692)
Total Assets	217,583	192,035	217,583	192,035
Goodwill	0	0	0	0
Cost of energy
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	472,465	433,586	1,163,736	1,113,458
Cost of energy	133,991	108,736	326,564	314,145
Utility margin	338,474	324,850	837,172	799,313
Cost of energy | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	364,504	331,113	873,425	838,649
Cost of energy	108,284	84,915	250,692	243,120
Utility margin	256,220	246,198	622,733	595,529
Cost of energy | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	107,961	102,473	290,311	274,809
Cost of energy	25,707	23,821	75,872	71,025
Utility margin	82,254	78,652	214,439	203,784
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