Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 400,023	$ 392,607	$ 1,042,033	$ 1,016,384
Revenues	422,666	419,900	1,092,857	1,112,398
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	306,019	300,604	783,310	769,069
Revenues	331,374	327,254	832,116	854,909
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	94,004	92,003	258,723	247,315
Revenues	91,292	92,646	260,741	257,489
Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	178,318		435,575
Residential | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	138,091		334,767
Residential | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	40,227		100,808
Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	150,605		399,340
Commercial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	121,755		315,256
Commercial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	28,850		84,084
Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	22,321		58,867
Industrial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	17,919		45,976
Industrial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,402		12,891
Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	8,262		20,931
Public authority | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,872		16,726
Public authority | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,390		4,205
Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,158		19,825
Economy energy service | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,158		19,825
Economy energy service | TNMP
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	30,281		90,123
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	13,538		40,128
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	16,743		49,995
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,078		17,372
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,686		10,632
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	2,392		6,740
Alternative revenue programs
Disaggregation of Revenue [Line Items]
Revenues	(8,050)	(1,908)	(1,466)	11,591
Alternative revenue programs | PNM
Disaggregation of Revenue [Line Items]
Revenues	(5,338)	(2,551)	(3,484)	1,417
Alternative revenue programs | TNMP
Disaggregation of Revenue [Line Items]
Revenues	(2,712)	643	2,018	10,174
Other electric operating revenues
Disaggregation of Revenue [Line Items]
Revenues	30,693	29,201	52,290	84,423
Other electric operating revenues | PNM
Disaggregation of Revenue [Line Items]
Revenues	30,693	$ 29,201	52,290	$ 84,423
Other electric operating revenues | TNMP
Disaggregation of Revenue [Line Items]
Revenues	$ 0		$ 0