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Electric Operating Revenue Narrative (Details)
Dec. 31, 2019
USD ($)
utility
MW
Dec. 31, 2018
USD ($)
Dec. 31, 2017
MW
Contract with Customers, Asset and Liability [Roll Forward]      
Number of regulated utilities | utility 2    
Capitalized contract cost, amortization period (in years) 5 years    
Contract assets | $ $ 0    
Public Service Company of New Mexico      
Contract with Customers, Asset and Liability [Roll Forward]      
Expected exposure to market risk (in megawatts) 65   65
Power to be sold to third party (in megawatts) 36   36
Contract with customers, net | $ $ 59,300,000 $ 61,700,000  
Palo Verde Nuclear Generating Station Unit 3 | Clean Air Act, SNCR | Public Service Company of New Mexico      
Contract with Customers, Asset and Liability [Roll Forward]      
Number of megawatts (in megawatts)     134