Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 455,120	$ 418,673	$ 1,121,177	$ 1,049,287
Alternative revenue programs	(12,376)	(6,779)	(7,484)	(300)
Other electric operating revenue	29,721	21,692	50,043	64,471
Revenues	472,465	433,586	1,163,736	1,113,458
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	341,850	311,793	825,759	775,795
Alternative revenue programs	(7,067)	(2,372)	(2,377)	(1,617)
Other electric operating revenue	29,721	21,692	50,043	64,471
Revenues	364,504	331,113	873,425	838,649
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	113,270	106,880	295,418	273,492
Alternative revenue programs	(5,309)	(4,407)	(5,107)	1,317
Other electric operating revenue	0	0	0	0
Revenues	107,961	102,473	290,311	274,809
Cost of energy
Disaggregation of Revenue [Line Items]
Revenues	472,465	433,586	1,163,736	1,113,458
Cost of energy | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	211,663	188,459	492,763	446,162
Cost of energy | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	156,491	152,744	392,995	392,432
Cost of energy | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	32,317	26,361	85,545	67,348
Cost of energy | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	9,450	7,580	21,547	19,651
Cost of energy | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,270	5,731	11,802	18,677
Cost of energy | PNM | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	158,549	137,741	370,448	331,371
Cost of energy | PNM | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	125,414	121,702	305,127	305,903
Cost of energy | PNM | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	25,475	20,946	65,313	51,022
Cost of energy | PNM | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	7,982	6,140	17,236	15,447
Cost of energy | PNM | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,270	5,731	11,802	18,677
Cost of energy | TNMP | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	53,114	50,718	122,315	114,791
Cost of energy | TNMP | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	31,077	31,042	87,868	86,529
Cost of energy | TNMP | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,842	5,415	20,232	16,326
Cost of energy | TNMP | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,468	1,440	4,311	4,204
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	37,308	33,452	103,822	92,766
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	17,463	16,068	45,727	43,794
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	19,845	17,384	58,095	48,972
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,621	4,346	12,703	12,251
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,697	3,465	10,106	9,581
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 924	$ 881	$ 2,597	$ 2,670