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Stock-Based Compensation - Performance Equity Plan and Accounting for Stock Awards (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
May 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense for stock-based arrangements $ 9.4 $ 8.1 $ 6.4  
Restricted Shares and Performance-Based Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Unrecognized compensation expense $ 4.4      
Period to recognize compensation expense 1 year 6 months      
Public Service Company of New Mexico        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense for stock-based arrangements $ 6.4 5.5 4.2  
Texas-New Mexico Power Company        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense for stock-based arrangements $ 3.0 $ 2.6 $ 2.2  
Performance Equity Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period for awards 3 years      
Vesting rate 100.00%      
Number of shares authorized (in shares)       13,500,000
Charge to share pool for each share awarded 5      
Performance Equity Plan | Nonemployee Members of the Board of Directors        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period for awards 1 year