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Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended
May 23, 2017
Mar. 31, 2017
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2014
Dec. 31, 2013
Income Tax Contingency [Line Items]            
New Mexico Corporate tax rate before change in 2013           7.60%
New Mexico Corporate tax rate, effective by 2018         5.90%  
Increase (decrease) in deferred tax assets as a result of tax rate changes       $ (0.7)    
Increase (decrease) in tax expense due to tax rate change (less than for 2017)       0.8    
Corporate and Other            
Income Tax Contingency [Line Items]            
Increase (decrease) in tax expense due to tax rate change (less than for 2017)   $ 0.1   (0.1)    
Internal Revenue Service (IRS)            
Income Tax Contingency [Line Items]            
Net refunds received from the IRS     $ 6.5      
Increase (decrease) in interest receivable     (2.1)      
Interest income     5.1      
Interest expense     0.7      
Professional fees     0.9      
Net pre-tax impacts, liability (refund)     (3.5)      
Internal Revenue Service (IRS) | Corporate and Other            
Income Tax Contingency [Line Items]            
Net pre-tax impacts, liability (refund)     (0.6)      
Internal Revenue Service (IRS) | PNM            
Income Tax Contingency [Line Items]            
Net pre-tax impacts, liability (refund)     (2.6)      
Internal Revenue Service (IRS) | Texas-New Mexico Power Company            
Income Tax Contingency [Line Items]            
Net pre-tax impacts, liability (refund)     $ 0.3      
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 3 years          
PNM            
Income Tax Contingency [Line Items]            
Increase (decrease) in regulatory liability   (4.8)   $ (7.1)    
Increase (decrease) in deferred tax assets as a result of tax rate changes   (0.1)        
Increase (decrease) in tax expense due to tax rate change (less than for 2017)   $ 0.1        
PNM | 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 3 years