Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 343,075	$ 314,917	$ 666,057	$ 630,614
Alternative revenue programs	4,466	5,844	4,892	6,480
Other electric operating revenue	10,108	9,467	20,322	42,778
Revenues	357,649	330,228	691,271	679,872
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	248,151	228,061	483,909	464,002
Alternative revenue programs	2,529	691	4,690	756
Other electric operating revenue	10,108	9,467	20,322	42,778
Revenues	260,788	238,219	508,921	507,536
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	94,924	86,856	182,148	166,612
Alternative revenue programs	1,937	5,153	202	5,724
Other electric operating revenue	0	0	0	0
Revenues	96,861	92,009	182,350	172,336
Cost of energy
Disaggregation of Revenue [Line Items]
Revenues	357,649	330,228	691,271	679,872
Cost of energy | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	146,392	119,968	281,099	257,701
Cost of energy | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	121,470	127,026	236,504	239,688
Cost of energy | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	27,230	20,624	53,228	40,987
Cost of energy | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,326	5,987	12,096	12,071
Cost of energy | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,278	6,024	8,531	12,946
Cost of energy | PNM | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	109,090	86,328	211,899	193,629
Cost of energy | PNM | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	93,364	98,968	179,713	184,201
Cost of energy | PNM | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	20,373	15,329	39,838	30,076
Cost of energy | PNM | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,907	4,596	9,254	9,307
Cost of energy | PNM | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,278	6,024	8,531	12,946
Cost of energy | TNMP | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	37,302	33,640	69,200	64,072
Cost of energy | TNMP | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	28,106	28,058	56,791	55,487
Cost of energy | TNMP | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,857	5,295	13,390	10,911
Cost of energy | TNMP | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,419	1,391	2,842	2,764
Cost of energy | TNMP | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	0	0	0	0
Transmission
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	34,335	31,927	66,514	59,316
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	14,097	14,342	28,264	27,727
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	20,238	17,585	38,250	31,589
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,044	3,361	8,085	7,905
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,042	2,474	6,410	6,116
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 1,002	$ 887	$ 1,675	$ 1,789