Segment Information - Summarized Financial Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments | segment			1
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	$ 554,551	$ 472,465	$ 1,345,798	$ 1,163,736
Other operating expenses	132,004	122,269	397,209	363,084
Depreciation and amortization	71,438	68,400	212,039	207,395
Operating income	151,729	147,805	269,098	266,693
Interest income	3,329	3,180	10,466	9,674
Other income (deductions)	2,536	14,062	16,864	2,759
Interest charges	(23,244)	(27,263)	(73,247)	(88,785)
Earnings before Income Taxes	134,350	137,784	223,181	190,341
Income taxes (benefit)	16,668	12,331	26,533	14,726
Net Earnings	117,682	125,453	196,648	175,615
Valencia non-controlling interest	(4,229)	(3,553)	(11,643)	(11,222)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	113,321	121,768	184,609	163,997
Total Assets	8,256,070	7,742,112	8,256,070	7,742,112	$ 7,939,854
Goodwill	278,297	278,297	278,297	278,297	$ 278,297
PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	107,385	99,171	320,638	299,284
Depreciation and amortization	42,673	40,509	127,111	123,721
Operating income	114,563	116,540	199,380	199,728
Interest income	3,364	3,239	10,376	9,882
Other income (deductions)	1,358	11,563	13,705	(1,509)
Interest charges	(12,509)	(14,747)	(38,441)	(51,554)
Earnings before Income Taxes	106,776	116,595	185,020	156,547
Income taxes (benefit)	15,500	13,591	26,178	16,127
Net Earnings	91,276	103,004	158,842	140,420
Valencia non-controlling interest	(4,229)	(3,553)	(11,643)	(11,222)
Subsidiary preferred stock dividends	(132)	(132)	(396)	(396)
Net Earnings Available for PNM Common Stock	86,915	99,319	146,803	128,802
Total Assets	5,723,176	5,449,238	5,723,176	5,449,238
Goodwill	51,632	51,632	51,632	51,632
TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	28,677	27,983	82,950	78,444
Depreciation and amortization	23,031	22,492	67,696	66,696
Operating income	38,842	31,779	80,571	69,299
Interest income	0	0	0	0
Other income (deductions)	1,144	2,286	3,251	4,810
Interest charges	(8,403)	(7,942)	(25,155)	(22,475)
Earnings before Income Taxes	31,583	26,123	58,667	51,634
Income taxes (benefit)	3,642	2,202	6,341	4,447
Net Earnings	27,941	23,921	52,326	47,187
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	27,941	23,921	52,326	47,187
Total Assets	2,311,559	2,075,291	2,311,559	2,075,291
Goodwill	226,665	226,665	226,665	226,665
Corporate and Other
Segment Reporting Information, Profit (Loss) [Abstract]
Other operating expenses	(4,058)	(4,885)	(6,379)	(14,644)
Depreciation and amortization	5,734	5,399	17,232	16,978
Operating income	(1,676)	(514)	(10,853)	(2,334)
Interest income	(35)	(59)	90	(208)
Other income (deductions)	34	213	(92)	(542)
Interest charges	(2,332)	(4,574)	(9,651)	(14,756)
Earnings before Income Taxes	(4,009)	(4,934)	(20,506)	(17,840)
Income taxes (benefit)	(2,474)	(3,462)	(5,986)	(5,848)
Net Earnings	(1,535)	(1,472)	(14,520)	(11,992)
Valencia non-controlling interest	0	0	0	0
Subsidiary preferred stock dividends	0	0	0	0
Net Earnings Available for PNM Common Stock	(1,535)	(1,472)	(14,520)	(11,992)
Total Assets	221,335	217,583	221,335	217,583
Goodwill	0	0	0	0
Cost of energy
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	554,551	472,465	1,345,798	1,163,736
Cost of energy	199,380	133,991	467,452	326,564
Utility margin	355,171	338,474	878,346	837,172
Cost of energy | PNM
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	435,523	364,504	1,030,685	873,425
Cost of energy	170,902	108,284	383,556	250,692
Utility margin	264,621	256,220	647,129	622,733
Cost of energy | TNMP
Segment Reporting Information, Profit (Loss) [Abstract]
Electric operating revenues	119,028	107,961	315,113	290,311
Cost of energy	28,478	25,707	83,896	75,872
Utility margin	90,550	82,254	231,217	214,439
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