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Regulatory and Rate Matters - Advanced Metering Infrastructure Application and Facebook Data Center Project (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2017
MW
May 12, 2017
USD ($)
Jul. 08, 2016
service_rate
MW
Feb. 26, 2016
USD ($)
Public Utilities, General Disclosures [Line Items]        
Number of requested service rates | service_rate     2  
Facebook Data Center        
Public Utilities, General Disclosures [Line Items]        
PPA term 25 years      
Public Service Company of New Mexico | Advanced metering infrastructure        
Public Utilities, General Disclosures [Line Items]        
Application to seek recovery of costs of project | $   $ 95.1   $ 87.2
Application to seek recovery of undepreciated investment | $       $ 33.0
PNMR Development        
Public Utilities, General Disclosures [Line Items]        
Solar distributed generation (in mw)     30  
Solar capacity (in mw)     10  
Casa Mesa Wind, LLC | Facebook Data Center        
Public Utilities, General Disclosures [Line Items]        
Solar distributed generation (in mw) 50      
Avangrid Renewables, LLC | Facebook Data Center        
Public Utilities, General Disclosures [Line Items]        
Solar distributed generation (in mw) 166      
Route 66 Solar Energy Center, LLC | Facebook Data Center        
Public Utilities, General Disclosures [Line Items]        
Solar distributed generation (in mw) 50