Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 322,983	$ 315,698
Alternative revenue programs	426	636
Other electric operating revenue	10,213	33,311
Revenues	333,622	349,645
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	235,759	235,941
Alternative revenue programs	2,161	66
Other electric operating revenue	10,213	33,311
Revenues	248,133	269,318
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	87,224	79,757
Alternative revenue programs	(1,735)	570
Other electric operating revenue	0	0
Revenues	85,489	80,327
Cost of energy
Disaggregation of Revenue [Line Items]
Revenues	333,622	349,645
Cost of energy | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	134,707	137,734
Cost of energy | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	115,034	112,662
Cost of energy | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	25,999	20,363
Cost of energy | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,770	6,085
Cost of energy | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,253	6,922
Cost of energy | PNM | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	102,809	107,302
Cost of energy | PNM | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	86,349	85,233
Cost of energy | PNM | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	19,466	14,747
Cost of energy | PNM | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,347	4,711
Cost of energy | PNM | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,253	6,922
Cost of energy | TNMP | Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	31,898	30,432
Cost of energy | TNMP | Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	28,685	27,429
Cost of energy | TNMP | Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,533	5,616
Cost of energy | TNMP | Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,423	1,374
Cost of energy | TNMP | Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	0	0
Transmission
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	32,179	27,388
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	14,167	13,385
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	18,012	14,003
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,041	4,544
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,368	3,641
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 673	$ 903