Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 314,917	$ 338,659	$ 630,614	$ 642,010
Operating revenues	330,228	352,313	679,872	670,191
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	228,061	254,728	464,002	477,291
Operating revenues	238,219	264,511	507,536	500,742
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	86,856	83,931	166,612	164,719
Operating revenues	92,009	87,802	172,336	169,449
Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	119,968	130,823	257,701	257,257
Residential | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	86,328	99,508	193,629	196,676
Residential | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	33,640	31,315	64,072	60,581
Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	127,026	138,734	239,688	248,735
Commercial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	98,968	110,652	184,201	193,501
Commercial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	28,058	28,082	55,487	55,234
Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	20,624	18,781	40,987	36,545
Industrial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	15,329	14,597	30,076	28,056
Industrial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,295	4,184	10,911	8,489
Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,987	6,619	12,071	12,670
Public authority | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,596	5,220	9,307	9,855
Public authority | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,391	1,399	2,764	2,815
Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,024	6,378	12,946	13,666
Economy energy service | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,024	6,378	12,946	13,666
Economy energy service | TNMP
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	31,927	30,851	59,316	59,841
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	14,342	14,108	27,727	26,590
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	17,585	16,743	31,589	33,251
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	3,361	6,473	7,905	13,296
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	2,474	4,265	6,116	8,947
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	887	2,208	1,789	4,349
Alternative revenue programs
Disaggregation of Revenue [Line Items]
Operating revenues	5,844	5,660	6,480	6,584
Alternative revenue programs | PNM
Disaggregation of Revenue [Line Items]
Operating revenues	691	1,789	756	1,854
Alternative revenue programs | TNMP
Disaggregation of Revenue [Line Items]
Operating revenues	5,153	3,871	5,724	4,730
Other electric operating revenues
Disaggregation of Revenue [Line Items]
Operating revenues	9,467	7,994	42,778	21,597
Other electric operating revenues | PNM
Disaggregation of Revenue [Line Items]
Operating revenues	9,467	7,994	42,778	21,597
Other electric operating revenues | TNMP
Disaggregation of Revenue [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0