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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
May 23, 2017
Jun. 30, 2019
Jun. 30, 2019
Income Tax Contingency [Line Items]      
Effective tax rate     8.93%
Tax benefits related to stock awards   $ 0.1 $ 0.8
TNMP      
Income Tax Contingency [Line Items]      
Effective tax rate     8.81%
Tax benefits related to stock awards   0.1 $ 0.2
PNM      
Income Tax Contingency [Line Items]      
Effective tax rate     11.20%
Tax benefits related to stock awards   $ 0.1 $ 0.5
NMPRC      
Income Tax Contingency [Line Items]      
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate 23 years    
NMPRC | PNM      
Income Tax Contingency [Line Items]      
Period of time for proposed return to customers the benefit of the reduction in New Mexico's corporate income tax rate 3 years