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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, 2016
MW
Dec. 31, 2019
USD ($)
GWh
Dec. 31, 2018
USD ($)
GWh
Dec. 31, 2017
USD ($)
GWh
Public Utilities, General Disclosures [Line Items]        
Hazard sharing agreement 1 year      
Agreement to sell the other party capacity and energy (in mw) | MW 100      
Number of hours sold (in GWh) | GWh   766.4 725.7 827.1
Hours sold (in dollars) | $   $ 21.7 $ 25.8 $ 23.6
Number of hours purchased (in GWh) | GWh   693.6 822.7 849.0
Hours purchased (in dollars) | $   $ 21.8 $ 28.7 $ 24.2