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Electric Operating Revenues - Narrative (Details)
 12 Months Ended
Dec. 31, 2020
USD ($)
MW
Dec. 31, 2019
USD ($)
Dec. 31, 2017
MW
Contract with Customers, Asset and Liability [Roll Forward]      
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) | MW 65   65
Power to be sold to third party (in megawatts) | MW 36   36
Contract with customers, net $ 86,200,000 $ 59,300,000  
Accounts receivable, credit loss expense (reversal) $ 7,200,000