Electric Operating Revenues - Disaggregation of revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 338,659	$ 326,586	$ 642,010	$ 623,777
Revenues	352,313	362,320	670,191	692,498
PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	254,728	246,402	477,291	468,465
Revenues	264,511	276,097	500,742	527,655
TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	83,931	80,184	164,719	155,312
Revenues	87,802	86,223	169,449	164,843
Residential
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	130,823		257,257
Residential | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	99,508		196,676
Residential | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	31,315		60,581
Commercial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	138,734		248,735
Commercial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	110,652		193,501
Commercial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	28,082		55,234
Industrial
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	18,781		36,545
Industrial | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	14,597		28,056
Industrial | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,184		8,489
Public authority
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,619		12,670
Public authority | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	5,220		9,855
Public authority | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	1,399		2,815
Economy energy service
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,378		13,666
Economy energy service | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,378		13,666
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	30,851		59,841
Transmission | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	14,108		26,590
Transmission | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	16,743		33,251
Miscellaneous
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	6,473		13,296
Miscellaneous | PNM
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,265		8,947
Miscellaneous | TNMP
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	2,208		4,349
Alternative revenue programs
Disaggregation of Revenue [Line Items]
Revenues	5,660	8,920	6,584	13,499
Alternative revenue programs | PNM
Disaggregation of Revenue [Line Items]
Revenues	1,789	2,881	1,854	3,968
Alternative revenue programs | TNMP
Disaggregation of Revenue [Line Items]
Revenues	3,871	6,039	4,730	9,531
Other electric operating revenues
Disaggregation of Revenue [Line Items]
Revenues	7,994	26,814	21,597	55,222
Other electric operating revenues | PNM
Disaggregation of Revenue [Line Items]
Revenues	7,994	$ 26,814	21,597	$ 55,222
Other electric operating revenues | TNMP
Disaggregation of Revenue [Line Items]
Revenues	$ 0		$ 0