XML 87 R65.htm IDEA: XBRL DOCUMENT v3.8.0.1
Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
May 23, 2017
Mar. 31, 2018
Mar. 31, 2016
Dec. 31, 2018
Dec. 31, 2014
Dec. 31, 2013
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 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      
Tax benefits related to stock awards   $ 1.3        
Corporate and Other            
Income Tax Contingency [Line Items]            
Increase (decrease) in tax expense due to tax rate change (less than for 2017)     (0.1)      
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          
TNMP            
Income Tax Contingency [Line Items]            
Increase (decrease) in regulatory liability   1.5        
Tax benefits related to stock awards   0.3        
PNM            
Income Tax Contingency [Line Items]            
Increase (decrease) in regulatory liability     $ (4.8)      
Tax benefits related to stock awards   $ 1.0        
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          
Forecast            
Income Tax Contingency [Line Items]            
Effective tax rate       12.11%    
Forecast | TNMP            
Income Tax Contingency [Line Items]            
Effective tax rate       23.47%    
Forecast | PNM            
Income Tax Contingency [Line Items]            
Effective tax rate       8.07%