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Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Income Tax Contingency [Line Items]      
New Mexico Corporate tax rate, 2014 7.60%    
New Mexico Corporate tax rate, effective by 2018     5.90%
Increase (decrease) in regulatory liability $ (7.1) $ (2.0)  
Impairment of state net operating loss carryforward 0.0 1.0  
Public Service Company of New Mexico [Member]      
Income Tax Contingency [Line Items]      
Increase (decrease) in deferred tax assets as a result of tax rate changes (0.7) 0.7  
Increase (decrease) in tax expense due to tax rate change 0.8 (0.5)  
Impairment of state net operating loss carryforward   0.7  
Texas-New Mexico Power Company [Member]      
Income Tax Contingency [Line Items]      
Impairment of state net operating loss carryforward   0.0  
Corporate and Other [Member]      
Income Tax Contingency [Line Items]      
Increase (decrease) in tax expense due to tax rate change $ (0.1) (0.2)  
Impairment of state net operating loss carryforward   $ 0.3