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Regulatory and Rate Matters - Advanced Metering Infrastructure Application and Facebook Data Center Project (Details)
$ in Millions
 3 Months Ended
Aug. 24, 2018
Facility
power_purchase_agreement
MW
Dec. 31, 2017
Dec. 31, 2018
MW
Sep. 05, 2017
MW
Aug. 17, 2016
service_rate
Jul. 08, 2016
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 25 years          
Number of additional PPAs | power_purchase_agreement 2            
Public Service Company of New Mexico | Advanced metering infrastructure              
Public Utilities, General Disclosures [Line Items]              
Application to seek recovery of costs of project | $             $ 87.2
Adjustment to estimated costs to be recovered | $             95.1
Application to seek recovery of undepreciated investment | $             $ 33.0
PNMR Development              
Public Utilities, General Disclosures [Line Items]              
Solar distributed generation (in mw)       50   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) 100   50        
Number of solar facilities | Facility 2