Electric Operating Revenues - Changes in contract liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Change In Contract With Customer Liability [Roll Forward]
Beginning balance	$ 349
Consideration received in advance of service to be provided	5,686
Deferred revenue earned	(4,438)
Ending balance	1,597
PNM
Change In Contract With Customer Liability [Roll Forward]
Beginning balance	349
Consideration received in advance of service to be provided	4,174
Deferred revenue earned	(3,304)
Ending balance	1,219
TNMP
Change In Contract With Customer Liability [Roll Forward]
Beginning balance	0
Consideration received in advance of service to be provided	1,512
Deferred revenue earned	(1,134)
Ending balance	$ 378