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Income Taxes (Details) - USD ($)
$ in Millions
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2021
Sep. 30, 2021
Mar. 31, 2018
Sep. 30, 2021
Dec. 31, 2017
Dec. 31, 2020
Income Tax Contingency [Line Items]            
Accelerated refund application amount of AMT tax, CARES act           $ 5.2
Deferred tax assets, tax deferred expense, payroll taxes, CARES Act           $ 7.0
Refund amount of AMT tax, CARES act $ 5.2          
Deferred federal, state and local, income tax expense (benefit)       $ 24.5    
Effective tax rate       13.08%    
Tax benefits related to stock awards   $ 0.1   $ 0.8    
Tax benefits on merger-related costs   0.3   1.8    
PNM            
Income Tax Contingency [Line Items]            
Deferred federal, state and local, income tax expense (benefit)       $ 15.2    
Effective tax rate       15.43%    
Tax benefits related to stock awards   0.1   $ 0.6    
Tax benefits on merger-related costs   0.1   0.1    
TNMP            
Income Tax Contingency [Line Items]            
Deferred federal, state and local, income tax expense (benefit)       $ 9.3    
Effective tax rate       11.12%    
Tax benefits related to stock awards   0.1   $ 0.2    
Tax benefits on merger-related costs   $ 0.1   $ 0.1    
NMPRC            
Income Tax Contingency [Line Items]            
Period of time for proposed return to customers the benefit of the reduction in federal corporate income tax rate         23 years  
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate     3 years