Regulatory and Rate Matters - Hazard Sharing Agreement (Details) - Public Service Company of New Mexico - Tri-State GWh in Millions, $ in Millions			12 Months Ended
	Jun. 01, 2017	Jun. 01, 2016 MW	Dec. 31, 2017 USD ($) GWh	Dec. 31, 2016 USD ($) GWh
Public Utilities, General Disclosures [Line Items]
Hazard sharing agreement	5 years	1 year
Agreement to sell the other party capacity and energy (in mw) | MW		100
Number of hours sold (in GWh) | GWh			827.1	482.3
Hours sold (in dollars) | $			$ 23.6	$ 12.8
Number of hours purchased (in GWh) | GWh			849.0	484.6
Hours purchased (in dollars) | $			$ 24.2	$ 12.9