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Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 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
Deferred federal, state and local, income tax expense (benefit) $ 24.5    
Effective tax rate 12.91%    
Tax benefits related to stock awards $ 0.7    
Tax benefits on merger-related costs 1.2    
PNM      
Income Tax Contingency [Line Items]      
Deferred federal, state and local, income tax expense (benefit) $ 15.2    
Effective tax rate 15.40%    
Tax benefits related to stock awards $ 0.5    
TNMP      
Income Tax Contingency [Line Items]      
Deferred federal, state and local, income tax expense (benefit) $ 9.3    
Effective tax rate 10.67%    
Tax benefits related to stock awards $ 0.2    
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