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Regulatory and Rate Matters - Electric Rate Case (Details) - Public Service Company of New Mexico [Member]
$ in Millions
 9 Months Ended
Aug. 29, 2015
USD ($)
Jul. 31, 2015
May. 27, 2015
Sep. 30, 2015
USD ($)
Jun. 25, 2015
utility
2014 Electric Rate Case [Member]          
Public Utilities, General Disclosures [Line Items]          
Public utilities, requested rate increase (decrease)       $ 107.4  
Public utilities, requested return on equity       10.50%  
Public utilities, average customer bill increase       7.69%  
Public utilities, percentage of requested rate increase pertaining to infrastructure investments       92.00%  
Future test year period, period of time following the filing of application for rate increase     45 days 13 months  
Public utilities, number of other utilities filing appeals (utilities) | utility         2
Requested stay period   30 days      
2015 Electric Rate Case [Member]          
Public Utilities, General Disclosures [Line Items]          
Public utilities, requested rate increase (decrease) $ 123.5        
Public utilities, requested return on equity 10.50%